Draft: RFC: Make app filtering a persistent setting

When using a distribution other than PureOS (such as Mobian), very few packages have the X-Purism-FormFactor property and application filtering is more of an annoyance as long as it hasn't been widely adopted by upstream projects (maintaining forks for the sole purpose of adding a property to the desktop file isn't an option for us due to scarce resources).

Instead of disabling app filtering completely (users may find it useful nonetheless), it would be interesting to have the filtering state persistent, so it can easily be enabled/disabled in a user-friendly way (i.e. not changing a gsetting value in the terminal) and remembered every time phosh is started.

This MR adds a gsetting (filter-app-list) to that effect as well as a new filter-enabled property for the app list. Filtering is therefore activated only if both filter-adaptive and filter-enabled are true, therefore honoring the gsetting while still following docked status when enabled.

There is still an issue with the current implementation: in docked mode, filtering will be disabled as expected, but the corresponding checkbox won't reflect that, I need to dig further into GAction in order to find a proper solution: ideally, the checkbox would reflect the value of filter-adaptive && filter-enabled, but toggling it should only change the value of filter-enabled.