Set reading font/size/margins/spacing/justification

[pgaskin]

This would be quite useful when combined with chain_* for having different presets to switch between. It would also allow setting sizes/margins/spacing outside the slider range, which can sometimes be useful.

I haven't looked into how feasible/stable/version-agnostic this would be yet. In particular, I need to look into when the reader view is updated, how this will interact with different file formats (i.e. if I can detect if the setting isn't supported for the current file format), and whether these actions will need to be disabled for non-reader menus.

An action to display the current settings will also be necessary to make it easier for users to figure out the current settings.

[pgaskin]

Some notes for myself:

• It appears that ReadingSettings::set* is all that's needed to set the settings.
• To refresh the view, ReadingMenuFontSettingsController::updateReadingView is called.
• I need to see if there's a safe and version-agnostic way to get ahold of the ReadingMenuFontSettingsController.
  • Hooking would always work (it's possible to safely check if a QObject is destroyed), but I don't want to add any additional hooks to NickelMenu itself.
  • The ReadingMenuFontSettingsController is created on-demand by ReadingMenuController::fontsMenu. It appears to be safe to create one separately, as long as there's some way to get a pointer to the current ReadingView.
  • I might be able to do something with the MainWindowController.

[pgaskin]

I've decided against doing this for now (it's possible, but I don't like how fragile it is [to make it less fragile, it would become quite a bit larger, and I don't think it's currently worth it] and nobody has requested this feature specifically yet anyways).