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/view.h

@@ -89,6 +89,7 @@ struct view {
 	bool mapped;
 	bool been_mapped;
 	bool ssd_enabled;
+	bool ssd_titlebar_hidden;
 	enum ssd_preference ssd_preference;
 	bool minimized;
 	bool maximized;