Add mnemonics to Boards menu: a-z,A-Z,0-9

[jaggzh]

Adds keystrokes to the Tools menu for (B)oards and (P)orts, as well is to those submenus themselves.

This is not only useful for switching more-quickly between projects, but also likely for accessibility, screen-readers, etc.

A couple functions have been added for selecting incremental letters/digits. They can be used for other menus as well.

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[jaggzh]