Add keyboard shortcut to run application.

Add keyboard shortcut to run application.

A common feature in several desktop environments is the ability to bring up
a "run application" dialog via a keyboard shortcut (frequently Mod1+F2).  This
feature has been available to Window Maker users only through the root menu.

This patch adds the ability for a user to set up a keyboard shortcut to
accomplish this, either via WPrefs or by editing ~/GNUstep/Defaults/WindowMaker,
e.g., with
 RunKey = "Mod1+F2";

Code from the execCommand function in rootmenu.c has been copied almost directly
into the handleKeyPress function in misc.c to accomplish this.