Implement assignments of workspaces to screens, big cleanup of workspace code

Please test this! Plug in screens, unplug them, use your video projector,
change resolutions, etc.

To use the assignments, use the following syntax:
workspace <number> [screen <screen>] [name]

Where screen can be one of:
<number> (It is not provided that these numbers stay constant, so use with care)
<x>x<y> (Coordinates where the screen starts, so 1280 will be fine to match the
	 screen right of the main screen if your main screen is 1280 pixels
	 width. However, 1281 will not match)
<x>
x<y>

Some examples follow:

 workspace 1 screen 0
 workspace 1 screen 1
 workspace 1 screen 1280x0
 workspace 2 screen 1280
 workspace 3 screen x0
 workspace 3 screen 1 www
 workspace 4 screen 0 mail

include/data.h

@@ -194,6 +194,13 @@ struct Workspace {
         /** Are the floating clients on this workspace currently hidden? */
         bool floating_hidden;
 
+        /** A <screen> specifier on which this workspace would like to be (if
+         * the screen is available). screen := <number> | <position> */
+        char *preferred_screen;
+
+        /** Temporary flag needed for re-querying xinerama screens */
+        bool reassigned;
+
         /** the client who is started in fullscreen mode on this workspace,
          * NULL if there is none */
         Client *fullscreen_client;