Re-jig options page (incl. remove "Restore-to-defaults" button)

Fixes #1200, and makes the following changes to the options page:

- Wholly remove the `Restore to defaults` button (see below).
- For non-empty text options, add an explanation that they are restored to their default value if left empty.
- Move *Custom key mappings* above the fold (it's one of the few options which isn't set-and-forget).
- For text options, activate the `Save` button as soon as text is changed (which avoids the need to leave the `textarea` before the button becomes active).

Regarding `Restore to defaults`:

- The utility of this button is limited because it permanently resets *all* options to their default value.  So it doesn't help recovering just a single broken option.
- I cannot think of a use case which isn't marginal.
- With this PR, we have an explanation of how to reset individual options.