gtk-3.14: invisible session properties dialog

Fix invisible session properties dialog when running with gtk-3.14

Closes https://github.com/mate-desktop/mate-session-manager/pull/63
Merge remote-tracking branch 'upstream/master'

Conflicts:
	capplet/gsm-app-dialog.c
	capplet/gsm-properties-dialog.c
Merge branch 'master' of github.com:dnk/mate-session-manager