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.

WPrefs.app/KeyboardShortcuts.c

@@ -147,6 +147,7 @@ static const struct {
 	/* Misc. */
 	{ "WindowRelaunchKey", N_("Launch new instance of application") },
 	{ "ScreenSwitchKey",   N_("Switch to Next Screen/Monitor") },
+	{ "RunKey",            N_("Run application") },
 	{ "DockRaiseLowerKey", N_("Raise/Lower Dock") },
 	{ "ClipRaiseLowerKey", N_("Raise/Lower Clip") }
 #ifdef XKB_MODELOCK