Qt (hotkeys): Context menu and modification highlighting

This complements !1344 (merged), adding to the hotkey editor:

• A context menu, notably with a reset option for resetting individual hotkey assignments. (Note that resetting assignments works correctly with respect to conflicts, raising a dialog, similar to the edit one, to ask how to proceed).
• Bold highlighting of modified entries.

The new features are built upon a bit of a code clean up that comes first, deriving from the fact that we needed to now keep track of additional data for each row.

Note that you can reset the entire table if the menu is raised from the action column, or otherwise entire individual columns.

Obligatory screenshot: