backend: Port to gtkglsink

• Use gtkglsink instead of clutter-gst
• Rotation support
• Navigation support
• Aspect ratio changes
• Popup behaviour
• Buffering spinner/progress bar
• Logo support
• Move video widgets to GtkBuilder
• Scrollwheel support
• Re-enable VAAPI plugins

Non-blocker:

• "Zoom" letterbox/crop support (moved to #497)
• Gestures support (moved to #498)
• Rotation "reset" (moved to #494 (closed))
• Bump version on rotation metadata name? (for when we fix #494 (comment 1367072))
• Better placeholder design (use speakers icon?) (moved to #495 (closed))
• Better buffering spinner design (moved to #474 (closed))

Drop:

• Showing cover for audio-only files
• Support for non-square pixels in output

Blocking bugs:

• gtkglsink rotation support
• gst-plugins-base rotation enums
• gst-plugins-base GL affine transformations
• gst-plugins-base broken GL pkg-config files
• gst-plugins-good gtkglsink rotation support
• gst-plugins-good gtksink coordinates translation
• gst-plugins-good gtksink video-aspect-ratio