Improve usability of mouse-keys mode

[inexorabletash]

Previous comments in #155
cc: @eschaton

Current behavior:

• To enter Mouse Keys mode, hold down △ and then press and release ▲.
• While holding △ down, you can you can navigate the cursor with the Arrow keys, and click using ▲.
• To exit Mouse Keys mode, release △.

Proposed behavior:

• Use a key shortcut to initiate Mouse Keys mode, like Easy Access
• Arrow keys plus (spacebar?) to click
• Another key shortcut to exit

[inexorabletash]

Good First Bug notes:

This will require diving into MGTK (mgtk/mgtk.s) and understanding how the current mouse keys mode works. Then tease it apart.

There are two options for triggering it - either leave it intrinsic to MGTK, or add a new MGTK call to invoke it and wire that up to DeskTop/Disk Copy/Selector. It's probably best to leave it intrinsic so that users can enter/exit the mode from nested event loops (e.g. while a dialog is up) without coding the shortcut everywhere. (We could make the activation shortcut configurable, though.)

Ideas for the shortcut to enter the mode are welcome. I was thinking △+▲+space or △+Esc. (pretend those are apples)

[inexorabletash]

To keep the code size down, the same sequence is used to enter/exit MouseKeys mode: △+▲+space. You no longer need to hold anything down. A sound is played on exit as well as enter.

▲ is used as the mouse button while the mode is active (no change).

[inexorabletash]

Revised:

• Enter MouseKeys mode: △+▲+space. A confirmation sound is played on enter.
• You no longer need to hold anything down to stay in MouseKeys mode.
• ▲ is used as the mouse button while the mode is active (no change).
• Exit MouseKeys mode: escape. A confirmation sound is played on exit.