mate-applets-1.5.1: bump EAPI, add charpick patch, misc fixes

Add patch to enable mucharmap, see [1] (need to remove in next version)
Remove gtk-doc(-am)
Remove die from src_test (> EAPI 4 emake dies on it's own)
Bump EAPI to 5
Remove gstreamer IUSE + mixer config option (moved to mate-media)

[1] https://github.com/mate-desktop/mate-applets/commit/79c1485d955e32a1d2fc64e96f8b342c76be3f4f