libmatekbd-1.5.1: Ebuild cleanup + bump EAPI

Remove obsolete comments
* Comment gtk+ 2.20 needed for test, minimum version available in portage is 2.24
* Remove commented src_prepare, all patches have been applied upstream

Bump EAPI to version 5.
* Add gtk3 to IUSE
* Add gtk+3 conditional DEP
* Mask gtk3 use flag for libmatekbd
mate-common-1.5.2: Ebuild clenaup

* >= EAPI 4 buildin functions die on their own
* May as well bump EAPI to 5.