Finished the zen mode functionality, and added full screen F11 shortcut

tests/reference/novelwriter.conf

@@ -1,5 +1,5 @@
 [Main]
-timestamp = 2019-11-08 18:43:21
+timestamp = 2019-11-09 13:48:06
 theme = default
 syntax = default_light
 guidark = False
@@ -9,6 +9,7 @@ geometry = 1100, 650
 treecols = 120, 30, 50
 mainpane = 300, 800
 docpane = 400, 400
+fullscreen = False
 
 [Project]
 autosaveproject = 60