Begin GTK 3.10 support.

This limits symbols deprecated in 3.10 to separate files requiring the
build tag gtk_3_8 before to be included in the build.  This allows
gotk3 to build on GTK 3.10 without erroring due to deprecated symbols.

This change adds no GTK 3.10 specific wrappers.
Implement wrappers for GtkSeparatorMenuItem
Merge remote-tracking branch 'main/master'