Set EWMH desktop properties on startup.

Calls ewmh_update_current_desktop on startup to set the
_NET_CURRENT_DESKTOP property. Without this change the property only
gets set after the workspaces have been manipulated. Also exclude
hidden workspaces (i.e. those starting with "__" from the workspace
index.

Adds tests for startup and workspace switching.

src/main.c

@@ -761,6 +761,9 @@ int main(int argc, char *argv[]) {
     x_set_i3_atoms();
     ewmh_update_workarea();
 
+    /* Set the _NET_CURRENT_DESKTOP property. */
+    ewmh_update_current_desktop();
+
     struct ev_io *xcb_watcher = scalloc(sizeof(struct ev_io));
     struct ev_io *xkb = scalloc(sizeof(struct ev_io));
     xcb_check = scalloc(sizeof(struct ev_check));