feat(developer): improve Touch Layout Editor and Character Map integration

Fixes #4854.

Adds some intelligence to the drag + drop integration with the Touch
Layout Editor, so that we always drop a character onto a key cap, we
always drop a code onto a ID field, and can drop both character + code
together (if the key id is T_new_..., or holding Ctrl key).

Hovering over any key for 0.25 sec while dragging will select that key.
You can then continue dragging down to the subkey array and drop onto
any of those keys. Note that you do not need to hover for 0.25 sec
before you drop onto a key -- the drop operation will select the key
under the cursor when dropping.

Double-clicking will update the selected key or subkey; Ctrl will do
both the key cap and the ID.