Avoid overriding OS X's built-in Command+` shortcut

OS X uses Command+\` for switching windows inside an application. If a Sublime package binds an operation to Command+\`, you cannot use the [standard OS X keyboard shortcut][1] to switch windows inside Sublime.

To avoid this conflict, this commit changes the `filename_status` keyboard shortcut to Control+Shift+\`.

[1]: http://support.apple.com/kb/ht1343