Issue #5: don't update stored settings when toggling checkbox on home screen.

We have a default scale menu in the Settings screen, so we don't need to make the
log/linear checkbox stick between sessions. This commit implements this, i.e. the
Settings screen is where you select the default scale on startup, and the checkbox
only toggles the scale during a session.
Issue #5 last commit for this issue, hopefully.

Restore saved settings when leaving home view (since I decided that changes
to settings outside of the "Settings" screen should be temporary).