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.

src/keybind.h

@@ -116,6 +116,9 @@ enum {
 	/* screen */
 	WKBD_SWITCH_SCREEN,
 
+	/* open "run" dialog */
+	WKBD_RUN,
+
 #ifdef KEEP_XKB_LOCK_STATUS
 	WKBD_TOGGLE,
 #endif