awesomerc: add keybinding to open menu with keygrabber

awesome.1.txt

@@ -84,6 +84,8 @@ Window manager control
     Run Lua code prompt.
 *Mod4 \+ Return*::
     Spawn terminal emulator.
+*Mod4 \+ w*::
+    Open main menu.
 
 Clients
 ~~~~~~~

awesomerc.lua.in

@@ -197,6 +197,7 @@ globalkeys =
             awful.client.focus.byidx(-1)
             if client.focus then client.focus:raise() end
         end),
+    key({ modkey,           }, "w", function () mymainmenu:show(true)        end),
 
     -- Layout manipulation
     key({ modkey, "Shift"   }, "j", function () awful.client.swap.byidx(  1) end),