txt::Cursor: corrected bounds checking in col_step().¹

1: Use of txt::Node::singular() was naïve. Also made col_extent()
safer by explicitly going to the extent instead of making a call,
and made col_abs() simply call col_step().
KeyMod: added modifier combinations.
ui: simplified use of UpdateActions, added flag_noclear.

ui::UpdateActions:
	added flag_noclear;
	added mask_flags;
	renamed mask_all to mask_actions;
	added mask_all for all enum members.

ui::Context:
	corrected masking in run_actions();
	made run_actions() clearer;
	added support for ui::UpdateActions::flag_noclear.
tty::Terminal: added explicit support for cursor + modifier permutations.¹

1: Tied to xterm, unfortunately. To support CapString::parm_*_cursor
properly, we'd need a parser for the ultra-arcane command language caps use.