Maintainence release of v8 series for packagers

• Require GTK3.14 for now (may be revisited if someone posts patches)
• Require Mutter 3.14
• Drop crappy workarounds
• Rewrite budgie-wm component
• Implement mediakeys integration with GNOME Settings Daemon
• Fix input grabs
• Better support of GNOME XDG configuration
• Modernised Run Dialog
• Refreshed panel editor UI
• Introduce MPRIS2 client (currently disabled in code for stability..)
• ^ That wasn't worth mentioning, was it?
• Lots of fixes and things I can't remember.
• This isn't v9.