Fixes for option to show hotkey names

* Adds the checkbox to menus/config.txt
* WidgetCheckBox's constructor does a call to mods->locate() already, so
passing the results of mods->locate() to the constructor caused a crash
* Use the SHOW_HOTKEYS boolean in MenuActionBar instead of maintaining
a private variable
* Fix the spelling of placeLabeledCheckbox()
Merge pull request #1 from dorkster/showkeynames

 Fixes for option to show hotkey names