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

docs/rc.xml.all

@@ -18,6 +18,7 @@
   <theme>
     <name></name>
     <cornerRadius>8</cornerRadius>
+    <keepBorder>yes</keepBorder>
     <font place="ActiveWindow">
       <name>sans</name>
       <size>10</size>