tree-wide: use rc.theme instead of server.theme

Having two global pointers to the same struct is redundant.

include/labwc.h

@@ -302,8 +302,6 @@ struct server {
 	/* Set when in cycle (alt-tab) mode */
 	struct cycle_state cycle;
 
-	struct theme *theme;
-
 	struct menu *menu_current;
 	struct wl_list menus;
 

include/theme.h

@@ -213,12 +213,9 @@ struct theme {
 	int mag_border_width;
 };
 
-struct server;
-
 /**
  * theme_init - read openbox theme and generate button textures
  * @theme: theme data
- * @server: server
  * @theme_name: theme-name in <theme-dir>/<theme-name>/labwc/themerc
  * Note <theme-dir> is obtained in theme-dir.c
  */