Persist the view mode setting

This change persists the view mode (Custom, Tree or Text). The setting will be saved whenever it is changed by selecting a new mode from the menu on the main window. 

It's a small change, but one that makes Robomongo much more pleasurable to use if you routinely use a view mode other than the default.

The change follows the pattern used by the already persistant LegacyUUIDEncoding setting.