clienticon: add a client setter (#2018)

It's required to work with the declarative syntax and be used
in the tasklist due to its limitations.

lib/awful/widget/clienticon.lua

@@ -31,7 +31,7 @@ end
 
 function clienticon:draw(_, cr, width, height)
     local c = self._private.client
-    if not c.valid then
+    if not c or not c.valid then
         return
     end
 
@@ -52,7 +52,7 @@ end
 
 function clienticon:fit(_, width, height)
     local c = self._private.client
-    if not c.valid then
+    if not c or not c.valid then
         return 0, 0
     end
 
@@ -80,6 +80,22 @@ function clienticon:fit(_, width, height)
     return w * aspect, h * aspect
 end
 
+--- The widget's @{client}.
+--
+-- @property client
+-- @param client
+
+function clienticon:get_client()
+    return self._private.client
+end
+
+function clienticon:set_client(c)
+    if self._private.client == c then return end
+    self._private.client = c
+    self:emit_signal("widget::layout_changed")
+    self:emit_signal("widget::redraw_needed")
+end
+
 --- Returns a new clienticon.
 -- @tparam client c The client whose icon should be displayed.
 -- @treturn widget A new `widget`