CI: update scripts to mirror libs-base setup

[triplef]

As suggested by Fred in gnustep/libs-base#285 (comment).

I basically took the updated libs-base scripts, removed the Windows section (GUI not supported yet), and added libs-base in dependencies.sh.

I also added the option for manual runs to use a different libs-base branch for testing.

Overall this mainly adds building GUI with libobjc2. I was wondering though, is anyone using the gnustep-1.9 runtime or could we remove this?

[triplef]

I also tried re-adding libs-back (with option to use different branch for manual runs), which seems to be required by some of the tests. This was previously part of the CI setup but was removed by @gcasa in b98a309 as part of #114, presumable because some tests started failing with that PR.

For now I just commented out building libs-back, but if someone can fix the tests we can then re-enable it (independent of this PR).

These are the tests that are currently failing with lib-back (full test logs can be downloaded from the CI run):

gui/NSNibLoading/basic.m:
Failed test:     basic.m:52 ... .gorm file was loaded properly using loadNibNamed:owner:topLevelObjects:
Failed test:     basic.m:58 ... .xib file was loaded properly using loadNibNamed:owner:topLevelObjects:
Failed test:     basic.m:64 ... .nib file was loaded properly using loadNibNamed:owner:topLevelObjects:

gui/NSPopUpButton/defaultSelection.m:
Failed set:        defaultSelection.m:15 ... problem in NSPopupButton GNUstep defaultSelection.

gui/NSSavePanel/setDelegate_reload.m:
Failed set:        setDelegate_reload.m:49 ... problem in NSSavePanel GNUstep setDelegate.

gui/NSView/NSView_autoresize_and_rounding.m:
Failed set:        NSView_autoresize_and_rounding.m:59 ... problem in NSView GNUstep autoresize_and_rounding.

gui/NSView/NSView_bounds_scale.m:
Failed set:        NSView_bounds_scale.m:97 ... problem in NSView GNUstep bounds_scale.

gui/NSView/NSView_convertRect.m:
Failed set:        NSView_convertRect.m:100 ... problem in NSView GNUstep converRect.

gui/NSView/NSView_frame_bounds.m:
Failed set:        NSView_frame_bounds.m:57 ... problem in NSView GNUstep frame_bounds.

gui/NSView/NSView_frame_rotation.m:
Failed set:        NSView_frame_rotation.m:21 ... problem in NSView GNUstep frame_rotation.

gui/NSView/scrollRectToVisible.m:
Failed set:        scrollRectToVisible.m:33 ... problem in NSView GNUstep scrollRectToVisible.

[fredkiefer]

Thank you! Looks great. I do an in detail review later on but as the three environments seem to work I don't expect to find any issues.