Missing unit action hotkeys

[elexis]

There are hotkeys for almost all unit actions (destroy, garrison, guard, ...), but some are missing, for example repairing and unloading. They should be added for completeness.

The most problematic part will be finding unused keys, as R is used for reseting the camera, see default.cfg. Maybe a modifier key has to be used.

The files to modify can be found in binaries/data/mod/public/gui/session/. unit_actions.js should show that hotkey in the tooltips, see also r18252.