Add options panel

[whimboo]

There are a couple of hard-coded values in the extension that warrants at least for now a preferences section within the Add-ons manager. Those are simple general values that should affect every open sidebar panel:

1. Interval that is used to fetch new process data. By default it is set to 5s, and should be configurable between 1s, 2s, and 5s. Maybe a custom value would make sense too.

2. Option to enable or disable the transfer of pages (windows) information. By default it should be enabled, but to save processing power it should be deselectable.

3. Option to enable or disable the transfer of threads information. By default it should be enabled, but to save processing power it should be deselectable.