Port GooCanvas to native GTK3/Cairo drawing

Replace all GooCanvas-based canvas widgets with GtkDrawingArea and
Cairo for rendering satellite tracking visualizations:

- gtk-polar-view: Polar plot for real-time satellite tracking
- gtk-polar-plot: Polar plot for pass visualization
- gtk-azel-plot: Azimuth/Elevation time plot
- gtk-sat-map: World map with satellite positions and ground tracks
- gtk-sky-glance: Timeline view of upcoming passes

Key changes:
- Replace GooCanvasItemModel objects with coordinate fields in structs
- Implement on_draw() callbacks using Cairo API for all rendering
- Add manual hit testing for mouse interactions
- Store render state (points, colors, positions) in widget structures
- Use gtk_widget_queue_draw() to trigger redraws

Build system updates:
- Remove goocanvas-2.0/3.0 pkg-config checks from configure.ac
- Add explicit gtk+-3.0 dependency check
- Update CI workflows and snap

This eliminates the external GooCanvas library dependency, reducing
build complexity and improving portability. The application now only
requires GTK+ 3.0 and standard system libraries.

Fixes: #369

Author: bgermann

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
         run: |
           sudo apt-get update
           sudo apt-get install -y build-essential libtool intltool autoconf automake libcurl4-openssl-dev \
-            pkg-config libglib2.0-dev libgtk-3-dev libgoocanvas-2.0-dev
+            pkg-config libglib2.0-dev libgtk-3-dev
 
       - name: Run autoconf
         run: ./autogen.sh

.travis.yml

@@ -9,7 +9,7 @@ compiler:
 before_install:
   - sudo apt-get update -qq
   - sudo apt-get install -y libtool intltool autoconf automake libcurl4-openssl-dev
-  - sudo apt-get install -y pkg-config libglib2.0-dev libgtk-3-dev libgoocanvas-2.0-dev
+  - sudo apt-get install -y pkg-config libglib2.0-dev libgtk-3-dev
 
 script:
   - ./autogen.sh

README

@@ -31,7 +31,6 @@ for successful compilation of Gpredict:
 
 - Gtk+ 3 or later
 - GLib 2.32 or later
-- GooCanvas 2 or 3
 - Libcurl 7.16 or later
 - Hamlib (runtime only, not required for build)
 
@@ -40,7 +39,7 @@ often with -dev or -devel in the package name, e.g. libgtk-3-dev. On Debian and
 Ubuntu systems you can install the build dependencies using:
 
   sudo apt install libtool intltool autoconf automake libcurl4-openssl-dev
-  sudo apt install pkg-config libglib2.0-dev libgtk-3-dev libgoocanvas-2.0-dev
+  sudo apt install pkg-config libglib2.0-dev libgtk-3-dev
 
 
 To build and install gpredict from source, first unpack the source package:

configure.ac

@@ -47,19 +47,12 @@ else
     AC_MSG_ERROR(Gpredict requires libglib-dev 2.40 or later)
 fi
 
-# check for goocanvas 2 or 3 (depends on gtk and glib)
-if $PKG_CONFIG --atleast-version=2.0 goocanvas-2.0; then
-    CFLAGS="$CFLAGS `$PKG_CONFIG --cflags goocanvas-2.0`"
-    LIBS="$LIBS `$PKG_CONFIG --libs goocanvas-2.0`"
-    havegoocanvas2=true
+# check for gtk+ 3.0 or later
+if $PKG_CONFIG --atleast-version=3.0 gtk+-3.0; then
+    CFLAGS="$CFLAGS `$PKG_CONFIG --cflags gtk+-3.0`"
+    LIBS="$LIBS `$PKG_CONFIG --libs gtk+-3.0`"
 else
-    if $PKG_CONFIG --atleast-version=3.0 goocanvas-3.0; then
-        CFLAGS="$CFLAGS `$PKG_CONFIG --cflags goocanvas-3.0`"
-        LIBS="$LIBS `$PKG_CONFIG --libs goocanvas-3.0`"
-        havegoocanvas3=true
-    else
-        AC_MSG_ERROR(Gpredict requires libgoocanvas-2.0-dev)
-    fi
+    AC_MSG_ERROR(Gpredict requires libgtk-3-dev)
 fi
 
 # check for libgps (optional)
@@ -103,12 +96,6 @@ GTHR_V=`$PKG_CONFIG --modversion gthread-2.0`
 GDK_V=`$PKG_CONFIG --modversion gdk-3.0`
 GTK_V=`$PKG_CONFIG --modversion gtk+-3.0`
 CURL_V=`$PKG_CONFIG --modversion libcurl`
-if test "$havegoocanvas2" = true ;  then
-    GOOC_V=`$PKG_CONFIG --modversion goocanvas-2.0`
-fi
-if test "$havegoocanvas3" = true ;  then
-    GOOC_V=`$PKG_CONFIG --modversion goocanvas-3.0`
-fi
 if test "$havelibgps" = true ; then
    GPS_V=`$PKG_CONFIG --modversion libgps`
 fi
@@ -148,7 +135,6 @@ echo Gio version........ : $GIO_V
 echo Gthread version.... : $GTHR_V
 echo Gdk version........ : $GDK_V
 echo Gtk+ version....... : $GTK_V
-echo GooCanvas version.. : $GOOC_V
 echo Libcurl version.... : $CURL_V
 if test "$havelibgps" = true ; then
    echo Libgps version..... : $GPS_V

snap/snapcraft.yaml

@@ -22,7 +22,6 @@ parts:
             - libgtk-3-dev
             - libcurl4-openssl-dev
             - libglib2.0-dev
-            - libgoocanvas-2.0-dev
         stage-packages:
             - libcanberra-gtk3-module