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/workspace.h

@@ -11,6 +11,7 @@
 #include <xcb/xcb.h>
 
 #include "data.h"
+#include "xinerama.h"
 
 #ifndef _WORKSPACE_H
 #define _WORKSPACE_H
@@ -32,4 +33,36 @@ void workspace_set_name(Workspace *ws, const char *name);
  */
 bool workspace_is_visible(Workspace *ws);
 
+/** Switches to the given workspace */
+void workspace_show(xcb_connection_t *conn, int workspace);
+
+/**
+ * Initializes the given workspace if it is not already initialized. The given
+ * screen is to be understood as a fallback, if the workspace itself either
+ * was not assigned to a particular screen or cannot be placed there because
+ * the screen is not attached at the moment.
+ *
+ */
+void workspace_initialize(Workspace *ws, i3Screen *screen);
+
+/**
+ * Gets the first unused workspace for the given screen, taking into account
+ * the preferred_screen setting of every workspace (workspace assignments).
+ *
+ */
+Workspace *get_first_workspace_for_screen(struct screens_head *slist, i3Screen *screen);
+
+/**
+ * Unmaps all clients (and stack windows) of the given workspace.
+ *
+ * This needs to be called separately when temporarily rendering a workspace
+ * which is not the active workspace to force reconfiguration of all clients,
+ * like in src/xinerama.c when re-assigning a workspace to another screen.
+ *
+ */
+void workspace_unmap_clients(xcb_connection_t *conn, Workspace *u_ws);
+
+
+void workspace_map_clients(xcb_connection_t *conn, Workspace *ws);
+
 #endif