Add keepBorder <theme> option and enable it by default

With the new keepBorder option enabled, the
ToggleDecorations action now has 3 states:

- the first time only disables the titlebar
- the second time disables the whole SSD
- the third time enables the whole SSD again

When the keepBorder action is disabled, the old 2-state
behavior is restored, e.g. the ToggleDecorations action
only toggles between on and off.

Fixes #813

include/config/rcxml.h

@@ -48,6 +48,7 @@ struct rcxml {
 	/* theme */
 	char *theme_name;
 	int corner_radius;
+	bool ssd_keep_border;
 	struct font font_activewindow;
 	struct font font_menuitem;
 	struct font font_osd;