MFH: r459693

Import code from mousepad development repository which fixes a dbus
messages storm causing heavy CPU usage, memory allocation and disk
usage when working with multiple windows.

While here also import a GTK3 specific fix.

Ref.:
https://bugzilla.xfce.org/show_bug.cgi?id=14184
https://bugzilla.xfce.org/show_bug.cgi?id=12134

Reported by: Erich Dollansky <freebsd.ed.lists@sumeritec.com>
Obtained from: https://git.xfce.org/apps/mousepad/

Approved by: ports-secteam (swills)