Update Desktop mode so the various settings do not conflict.

[bluemarvin]

In FxR there are two ways to set the desktop mode: via the setting/display dialog and then via a button on the nav bar. The settings/display dialog is supposed to be global while the nav bar button is supposed to provide a per window override. However changing the global setting doesn't affect the local buttons visual state. The desktop mode should be changed to work as follows:

1. Changing the value in the setting/display dialog only affects new windows opened after it is changed and does not affect currently opened windows.
2. The nav bar button is used to toggle the focused window between desktop and VR mode.

[bluemarvin]

Fixed by #2069