Add hotkey support to labels and groupboxes

[mooffie]

Labels and groupboxes should support hotkeys in their text. Pressing the hotkey will move the focus to the following focus-able widget (e.g., input, listbox, radio). This is like <label> in HTML.

To understand why we need this, look for example at the Find File (and other) dialog:

Start at:
______________________________

File name:
______________________________

[ ] Some checkbox
[ ] Another checkbox

Content:
______________________________

[ ] Yet another checkbox

The problem here is that there's no way to move the focus directly to the input fields short of tabbing to them.

Here's how we can solve the problem with hotkey-capable labels:

This is a mockup screenshot. The new hotkeys have green arrows pointing to them. So, for example, pressing alt-e will take you to the Content field.

Other places to see the problem:

• The "Advanced chown" dialog (you can't jump directly to a button).
• The "Virtual File System Setting" dialog.
• mc²'s "Visual Rename" dialog:

(Above, you can't easily return to the "Pattern:" or "Replace with:" fields once you leave them.)

Groupboxes

The same with groupboxes. Their labels should support hotkeys so that pressing them will move you to the first focus-able widget "within". Examples:

• The "Chown" dialog (two listboxes inside groupboxes).
• The "External panelize" dialog (just one listbox here).

Admittedly those two aren't very convincing examples, so here's one from mc²:

None of the input fields there ("Output filename", "Aditional convert(1) args", etc.) have hotkeys that lead to them.