How hotkeys are specified should be consistent: Use underscore _

[tig]

• Button uses the first uppercase character
• MenuItem uses the underscore (_) character
• CheckBox uses _
• RadioGroup uses _
• StatusBar uses parameter to StatusItem, and the Shortcut property (Key). (this is OK).

This should be made consistent.

In addition, they way Button works means there's no way to say "NO HOTKEY!" if you without doing all lower case, which is sub-optimal.

Proposal:

• Change all controls to use the same logic MenuItem uses, which is to have the first instance of an underscore indicate the following char is the hotkey.
• Ensure that a double underscore (__) means to treat the underscore as a literal.

See #455

[tig]

PR #590 fixes this for Button

[tig]

This is now consistent across all controls (except StatusBar, but that's ok because it supports things like F1).