widget: Remove unused tint colour
widget: Add assertions to public API

Move the assertions to the public API instead of having them in the private
functions.
a11y: Use correct function to compare colours

Can't compare PangoColors with gdk_color_equal!
widget: Add more keys that the IM may need
keymap: Add missing modifiers to the list
keymap: Add missing Begin key to list of cursor keys
vteapp: Plug a mem leak

==27552== 10 bytes in 1 blocks are definitely lost in loss record 1,262 of 7,046
==27552==    at 0x4027AB1: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==27552==    by 0x4B17C32: g_malloc (gmem.c:97)
==27552==    by 0x4B30260: g_strdup (gstrfuncs.c:356)
==27552==    by 0x4057921: vte_get_user_shell (vte.c:3464)
==27552==    by 0x804BFC4: main (vteapp.c:1014)
a11y: Use new gtk a11y infrastructure

Derive VteTerminalAccessible from GtkWidgetAccessible, and use instance
private data instead of gobject data for VteTerminalPrivate.
a11y: Remove atk component get_{extends,size,position} impls

GtkWidgetAccessible's implementation of these is the same already.
a11y: Remove duplicated code

Remove code copied from GtkWidgetAccessible.

This reverts commit 3bdb0639aa26ce1dcd745bab71a6bd5dddca6019.
widget: Remove drawable checks
widget: Install default CSS at FALLBACK priority
widget: Add style debug flag

... and remove the unused background debug flag.

Conflicts:
	src/debug.c
	src/debug.h
vteapp: Add options to set highlight colour
a11y: Don't explicitly set accessible parent

Most other widget's a11y implementations don't do this either.