keygrabber: Add a new signal to catch awful.keys activations.

Some downstream modules with keys don't control the callbacks. Using
signals on multiple keys is more cumbersome than simply exposing
this at the keygrabber level.

lib/awful/keygrabber.lua

@@ -236,11 +236,18 @@ local function runner(self, modifiers, key, event)
                 for _,v2 in ipairs(v.modifiers) do
                     match = match and mod[v2]
                 end
-                if match and v.on_press then
-                    v.on_press(self)
 
-                    if self.mask_event_callback ~= false then
-                        return
+                if match then
+                    self:emit_signal("keybinding::triggered", v, event)
+
+                    if event == "press" and v.on_press then
+                        v.on_press(self)
+
+                        if self.mask_event_callback ~= false then
+                            return
+                        end
+                    elseif event == "release" and v.on_release then
+                        v.on_release(self)
                     end
                 end
             end
@@ -630,6 +637,12 @@ end
 --- When the keygrabber stops.
 -- @signal stopped
 
+--- When an `awful.key` is pressed.
+-- @signal keybinding::triggered
+-- @tparam awful.keybinding self
+-- @tparam awful.key key The keybinding.
+-- @tparam string event Either `"press"` or `"release"`.
+
 --- A function called when a keygrabber starts.
 -- @callback start_callback
 -- @tparam keygrabber keygrabber The keygrabber.