Switch to version 3.0 in a lot of places and depend on GTK+ 3.2

and lots of other newer libraries.
libgimpwidgets: remove deprecated files from the build
libgimpwidgets: s/GtkObject/GtkAdjustment/ in gimpwidgets.[ch]
libgimpwidgets: port virtual functions to GTK+ 3.0
libgimpwidgets: port virtual functions to GTK+ 3.0
libgimpwidgets: port to GtkWidget::draw()
libgimpwidgets: port to GtkWidget::draw()
libgimpwidgets: port to GtkWidget::draw()
libgimpwidgets: s/GtkObject/GtkAdjustment/
libgimpwidgets: port to GtkWidget::draw()
libgimpwidgets: remove deprecated functions and s/GtkObject/GtkAdjustment/
libgimpwidgets: remove deprecated files completely
libgimp: remove GdkRGB and GdkColorMap calls, these APIs are gone
app: port to GtkWidget::draw()
app: remove GTK_OBJECT() cast
app: port to GtkWidget::draw()
app: port to GtkWidget::draw()
app: port virtual functions to GTK+ 3.0 API
app: get rid of a GTK_OBJECT() cast
app: port to GtkWidget::draw()