Add PySide2 support to pygui (#649)

* Add PySide2 support to pygui

* change pyside2 to :qt5 and add :qt_pyside2

* refactor codes

Author: cherichy

src/gui.jl

@@ -24,11 +24,13 @@ pygui_works(gui::Symbol) = gui == :default ||
      (gui == :gtk && pyexists("gtk")) ||
      (gui == :gtk3 && pyexists("gi")) ||
      (gui == :tk && pyexists(tkinter_name())) ||
-     ((gui == :qt5 || gui == :qt_pyqt5) && pyexists("PyQt5")) ||
      (gui == :qt_pyqt4 && pyexists("PyQt4")) ||
+     (gui == :qt_pyqt5 && pyexists("PyQt5")) ||
      (gui == :qt_pyside && pyexists("PySide")) ||
+     (gui == :qt_pyside2 && pyexists("PySide2")) ||
      (gui == :qt4 && (pyexists("PyQt4") || pyexists("PySide"))) ||
-     (gui == :qt && (pyexists("PyQt5") || pyexists("PyQt4") || pyexists("PySide"))))
+     (gui == :qt5 && (pyexists("PyQt5") || pyexists("PySide2"))) ||
+     (gui == :qt && (pyexists("PyQt5") || pyexists("PyQt4") || pyexists("PySide") || pyexists("PySide2"))))
 
 # get or set the default GUI; doesn't affect running GUI
 """
@@ -53,7 +55,7 @@ end
 function pygui(g::Symbol)
     global gui
     if g != gui::Symbol
-        if !(g in (:wx,:gtk,:gtk3,:tk,:qt,:qt4,:qt5,:qt_pyqt5,:qt_pyqt4,:qt_pyside,:default))
+        if !(g in (:wx,:gtk,:gtk3,:tk,:qt,:qt4,:qt5,:qt_pyqt5,:qt_pyqt4,:qt_pyside,:qt_pyside2,:default))
             throw(ArgumentError("invalid gui $g"))
         elseif !pygui_works(g)
             error("Python GUI toolkit for $g is not installed.")
@@ -155,7 +157,7 @@ qt_eventloop(QtModule::AbstractString, sec::Real=50e-3) =
     qt_eventloop(pyimport("$QtModule.QtCore"), sec)
 
 function qt_eventloop(sec::Real=50e-3)
-    for QtModule in ("PyQt5", "PyQt4", "PySide")
+    for QtModule in ("PyQt5", "PyQt4", "PySide", "PySide2")
         try
             return qt_eventloop(QtModule, sec)
         catch
@@ -227,18 +229,26 @@ function pygui_start(gui::Symbol=pygui(), sec::Real=50e-3)
             eventloops[gui] = gtk_eventloop("gi.repository.Gtk", sec)
         elseif gui == :tk
             eventloops[gui] = Tk_eventloop(sec)
-        elseif gui == :qt_pyqt5 || gui == :qt5
-            eventloops[gui] = qt_eventloop("PyQt5", sec)
         elseif gui == :qt_pyqt4
             eventloops[gui] = qt_eventloop("PyQt4", sec)
+        elseif gui == :qt_pyqt5
+            eventloops[gui] = qt_eventloop("PyQt5", sec) 
         elseif gui == :qt_pyside
             eventloops[gui] = qt_eventloop("PySide", sec)
+        elseif gui == :qt_pyside2
+            eventloops[gui] = qt_eventloop("PySide2", sec)
         elseif gui == :qt4
             try
                 eventloops[gui] = qt_eventloop("PyQt4", sec)
             catch
                 eventloops[gui] = qt_eventloop("PySide", sec)
             end
+        elseif gui == :qt5
+            try
+                eventloops[gui] = qt_eventloop("PyQt5", sec)
+            catch
+                eventloops[gui] = qt_eventloop("PySide2", sec)
+            end
         elseif gui == :qt
             eventloops[gui] = qt_eventloop(sec)
         else