Merge branch 'master' into 'gtk3'

Most conflicts seemed to be simple enough. There were a lot of `append_text` to
`append` conversions for `Gtk::ComboBoxText`. The `PopUpCommon` class also saw
a lot of changes with non-trivial conflict resolutions.

rtgui/diagonalcurveeditorsubgroup.cc

@@ -30,6 +30,7 @@
 #include "mydiagonalcurve.h"
 #include "curveeditor.h"
 #include "diagonalcurveeditorsubgroup.h"
+#include "rtimage.h"
 
 DiagonalCurveEditorSubGroup::DiagonalCurveEditorSubGroup (CurveEditorGroup* prt, Glib::ustring& curveDir) : CurveEditorSubGroup(curveDir)
 {