Track and free idle task tags properly. Change F11 and F12 capabilities

Track and free idle task tags properly. Change F11 and F12 capabilities

* src/vte.c: Track and free idle task tags properly.  Change F11 and F12
	capabilities from 'k;' and 'F1' to 'F1' and 'F2'.  Send a NUL on
	control space. (#80350)  Allow setting and checking of word characters,
	and change select-by-word behavior to use the word character list.  Emit
	"contents_changed" signals whenever the visible contents change, and
	"cursor_moved" when the cursor moves.  Add snapshotting method.  Scroll
	when auto-margin handling moves the cursor to the next line.  Assume
	that the locale charset is actually ISO-8859-1 when we're in a UTF-8
	locale, so we don't toggle from UTF-8 to UTF-8.  Treat GDK_KP_Page_Up
	as a GDK_Page_Up, ditto for GDK_KP_Page_Down and GDK_KP_Tab and
	GDK_KP_Space.  Add vte_terminal_get_font().  Don't bother messing with
	ring buffers if we're resizing them to their current sizes.
* src/pty.c, src/vte.c: Return a pid from vte_terminal_fork_command().
* src/vteaccess.c, src/vteaccess.h: Add VteTerminalAccessible object type.  It
	might even work, mostly.