Hotkey editor removes unused hotkeys.

[nani]

When the hotkey editor saves the new hotkeys states, it removes the ones that don't have any mapping assigned, so if you add a hotkey not in default.cfg it will be removed from user.cfg making it dissapear forever.

Case and example:
1) mod adds extra hotkeys
2) user removes mapping to of one of the mods hotkeys
3) save
4) hotkey will no longer show in the hotkey editor ever

Proposal fix: in the hotkey editor have an internal state for each hotkey so it can know if is it in default.cfg (thus can safe delete) or otherwise don't delete it.

[wraitii]

This can be fixed following ​Phab:D3641, though I don't think that diff fixes that just yet.

[Stan]

In r25175:

Show non-default hotkeys that appear in JSON spec files in the hotkey editor.