We toggle in mate-settings-daemon now.
Also remove from EXTRA_DIST
lockdown: Move disable-log-out key here from mate-panel

To be used in more places like mate-session.
mate-about: Add Google Summer of Code students
mate-about: Add MATE Slackware maintainers
Sync translations with transifex
mate_desktop_gtk_style_get_light_color and mate_desktop_gtk_style_get_dark_color
gtk3: append missing include
gtk3: rgba rr-labeler api