Fix three keyboard-related issues (#13787)

[H09-0]

Summary

Fix #13787 - three keyboard and shortcut issues in the modal event handler
and menu search field.

---

Issue 1: Ctrl+Shift order-dependent for layout switching

Root cause: In _stageEventHandler (js/ui/main.js), when the user
presses two modifier keys (e.g., Ctrl+Shift) and the keybinding is stored
as <Control>Shift, only "Ctrl first, then Shift" triggers the binding.
"Shift first, then Ctrl" fails because get_keybinding_action(ctrl_keycode, Shift_mask) checks for a binding where Ctrl is the trigger key, but the
binding has Shift as the trigger key.

Fix: Track the previous modifier keycode (_lastModifierKeyCode) when a
modifier press doesn't match a binding. When the next modifier is pressed,
try the reverse combination: get_keybinding_action(prev_keycode, modifierState | current_mask). This correctly matches <Control>Shift
when Shift is pressed first.

Issue 2: Super+Space closes the Start Menu

Root cause: When Super (the overlay key) is pressed, _stageEventHandler
stores its action in _modifierOnlyAction, deferred to key-release. If the
user then presses another modifier key that doesn't match a binding,
_modifierOnlyAction was never cleared, causing the overlay toggle to fire
on Super release - closing the menu while the user intended Super as part
of a combo (Super+Space for layout switching).

Fix: Clear _modifierOnlyAction in the modifier key press handler when
the press doesn't match any binding. Also clear it when a non-modifier key
press arrives. This ensures a subsequent modifier-only action only fires if
no other key was pressed between press and release.

Issue 3: Ctrl+A does not select all text in search field

Root cause: The menu applet's _onMenuKeyPress handler connects to the
search entry clutter_text's key-press-event. ClutterText does not
implement Ctrl+A (select all) by default in the version used by Cinnamon.
The event propagated but no built-in handler acted on it.

Fix: Add Clutter.KEY_A / Clutter.KEY_a cases in the _onMenuKeyPress
switch statement. When ctrlKey is held, call
this.searchEntryText.set_selection(0, textLen) to select all text.

---

Files changed

• js/ui/main.js (+51, -5): Fix Issues 1 and 2 in _stageEventHandler
• files/usr/share/cinnamon/applets/menu@cinnamon.org/applet.js (+8, -0): Fix Issue 3 in _onMenuKeyPress

Testing

1. Issue 1: Set Ctrl+Shift as layout switching shortcut. With the menu
   open, press Shift first then Ctrl (both orders). Layout should switch
   regardless of order.
2. Issue 2: Open the Start Menu (by mouse click). Press Super+Space
   (or whatever the layout switch combo is). The menu should stay open
   and the layout should switch. Solo Super press/release should still
   toggle the menu.
3. Issue 3: Open the Start Menu, type text in the search field, press
   Ctrl+A. All text should be selected.

[H09-0]

Edge-case review completed

I reviewed the 5 concerns raised during review. Two real bugs were found and fixed in commit 12717ad:

1. Reverse-match modifier state was wrong: The old code passed modifierState | currentMask as the modifier state for the reverse lookup. Since modifierState already contains the held modifier's mask (whose keycode is _lastModifierKeyCode), this created a self-referential mismatch for bindings like <Control>Shift. Fixed to pass only currentMask.

2. Stale _lastModifierKeyCode across modal sessions: Added _lastModifierKeyCode = 0 to _completeModalSetup() to prevent stale keycodes from causing false reverse matches when a new modal session starts.

All 5 originally flagged concerns were confirmed as already correct: cleanup on release/non-modifier keys, _modifierOnlyAction clearing on successful combos, three-modifier graceful degradation, left/right modifier distinction, and Ctrl+A ctrlKey-only strictness.