Add zoom options to context menu.

Resolves #1213

(Alternatively could go in main menu (from the top-bar) but I feel it's crowded enough already (unless we have nested menus available).)

data/themes/default.cfg

@@ -118,6 +118,8 @@
                 "wbexecuteaction,wbdeleteaction,wbbumpupaction,wbbumpdownaction,wbsupposedead," +
                 # Information
                 "describeterrain,describeunit,renameunit," +
+                # Zoom
+                "zoomin,zoomout,zoomdefault," +
                 # Debug commands
                 "createunit,changeside,killunit," +
                 # Labels