Changes to show hidden files preference, calculations in file properties window

Changed the behavior for displaying hidden files:

- Removed the Show Hidden toggle from preferences.  Added it to the right-click menu.
- Toggling show hidden now always updates the gsettings key, as well as the views in the current window.  Other windows are unaffected.  Any new windows will use the gsettings key for their initial state.

Changed the behavior of the file properties window:

- When show hidden is set, all files and folders are counted -  "x items"
- When show hidden is unset, the hidden files and folders are counted separately, displaying "x items (and y hidden items)"
- The total size calculation will *always* include hidden files, regardless of the show hidden state.