BOptionPopUp: build fix.
Add SerialConnect to DeskBar menu.

It works well enough now.
Keymap preferneces: improve deadkey handling logic

* The key down message is not delivered when completing a dead key, so
act on key up instead.
* Pressing a character after a dead key would leave the keymap in "dead
key" mode, and that mode would be exited only by pressing a dead key
again.

Fixes #5734.