Support paging in SettingsPanel

[protyposis]

Current state

Different setting groups need to be added into different SettingsPanel instances, and these panels need to be switched on/off to switch between the setting groups.

Example: SettingsToggleButton opens a SettingsPanel with a SubtitleSettingsOpenButton that opens another SettingsPanel instance, the SubtitleSettingsPanel. The SubtitleSettingsOpenButton hides the SettingsPanel and shows the SubtitleSettingsPanel, and the SubtitleSettingsCloseButton does the inverse. This results in a odd UX and has the side effect that the SettingsToggleButton is only in the active state while its "own" SettingsPanel` is shown, but it does not know anything about "sub-settings panels".

Desired state

A SettingsPanel should be a container for multiple pages/panes, which can be navigated. In our example, we could have a MainSettingsPage and a SubtitleSettingsPage within the SettingsPanel and switch between them with navigation actions (e.g. via API or buttons). There could even be nice animated transitions. The UX would be nicer and the problem with the wrongly inactive SettingsToggleButton solved too.

[protyposis]

Released in 3.0.0