feat(settings): a Shortcuts tab that lists every keybind and lets you change them

[minecart551-prog]

feat(settings): a Shortcuts tab that lists every keybind and lets you set one
The editor's chords lived in the .slint tree, one KeyBinding apiece,
where nobody could see them together or change one. They are Rust's
now: keymap.rs names every action and the chord it ships with, and
Settings > Shortcuts lists them with what they answer to. Click a row
and press the keys it should take; Escape cancels. A chord another
action holds moves over to its new owner, leaving the old one unbound
until that row's Reset puts the shipped chord back, or Reset all
starts over. An unbound key does nothing.

A few chords are the window's own (Ctrl+O/W/Q, F10, Alt+F/E/V for the
menus) and are refused with a word saying so. The tray's labels and
tooltips read the same map, so a rebound chord shows there too; the
four tray strings that spelled out their key take it from {0} now, the
way undo and redo already did. Q and E's cut rows come with the list,
and every locale carries the new lines.

feat(timeline): Q and E cut the selected clip at the playhead
Q drops the clip's left up to the playhead and E its right from there
on - the trim keys the big editors have - each as one undo step. The
cut only happens when the playhead sits inside a selected clip on an
unlocked track, so a press with nothing under it says so rather than
chewing a neighbour. The lane closes behind the edge only on a magnetic
timeline, and trim follow carries the playhead to a head cut's new
start.

Signed-off-by: minecart551-prog minecart551@gmail.com