Add functions for calling something for every screen

This adds gears.screen which contains a wrapper around
screen.connect_signal("added", func) that also calls the callback function for
each screen that already exists. This is added in gears.screen so that it can
also be used from e.g. wibox, if needed. Feel free to move this elsewhere if
that's a bad idea (I'm not really convinced of it).

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/gears/init.lua

@@ -18,6 +18,7 @@ return
     matrix = require("gears.matrix");
     shape = require("gears.shape");
     protected_call = require("gears.protected_call");
+    screen = require("gears.screen");
 }
 
 -- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80

lib/gears/screen.lua

@@ -0,0 +1,29 @@
+---------------------------------------------------------------------------
+-- @author Uli Schlachter
+-- @copyright 2016 Uli Schlachter
+-- @release @AWESOME_VERSION@
+-- @classmod gears.screen
+---------------------------------------------------------------------------
+
+local screen = screen
+
+local module = {}
+
+--- Call a function for each existing and created-in-the-future screen.
+-- @tparam function func The function to call.
+function module.connect_for_each_screen(func)
+    for s in screen do
+        func(s)
+    end
+    screen.connect_signal("added", func)
+end
+
+--- Undo the effect of connect_for_each_screen.
+-- @tparam function func The function that should no longer be called.
+function module.disconnect_for_each_screen(func)
+    screen.disconnect_signal("added", func)
+end
+
+return module
+
+-- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80