Refactor workspaces to be stored in a TAILQ instead of an array

This fixes many problems we were having with a dynamically growing
array because of the realloc (pointers inside the area which was
allocated were no longer valid as soon as the realloc moved the
memory to another address).

Again, this is a rather big change, so expect problems and enable
core-dumps.

include/data.h

@@ -232,6 +232,8 @@ struct Workspace {
          * opened, for example) have the same size as always */
         float *width_factor;
         float *height_factor;
+
+        TAILQ_ENTRY(Workspace) workspaces;
 };
 
 /**
@@ -492,7 +494,7 @@ struct Screen {
         int num;
 
         /** Current workspace selected on this virtual screen */
-        int current_workspace;
+        Workspace *current_workspace;
 
         /** x, y, width, height */
         Rect rect;