Add keyboard shortcuts for menus
See https://github.com/OpenMW/openmw/pull/1481 for the new features. Original request preserved below ================
Simple things, like pressing the l key to Load a game or the e key to Exit, and more complex things, like tabbing through a list of settings and using arrow keys or numbers to change a value. It shouldn't be necessary to use a mouse when, for some users, a keyboard would be faster and more comfortable.