Set up the preferences panel to have a browser selector that:
- Has a checkbox that makes Dash Redirector the default browser and forwards calls onto a fallback browser.
- Selects the default fallback browser when Dash Redirector is the active browser.
Set up the redirector to use the fallback browser that was configured in the preferences window.
Set up the preferences window to show and hide the fallback browser selector when appropriate.
Merge branch 'feature/pref-panel-default-browser'

* feature/pref-panel-default-browser:
  Set up the preferences window to show and hide the fallback browser selector when appropriate.
  Set up the redirector to use the fallback browser that was configured in the preferences window.
  Set up the preferences panel to have a browser selector that: - Has a checkbox that makes Dash Redirector the default browser and forwards calls onto a fallback browser. - Selects the default fallback browser when Dash Redirector is the active browser.