Invert Double Page switch

[rickymcmuffin]

I added a toggle that switches which page comes first in double page mode.

Before:

After:

Here's with it off:

Here's with it on:

It is currently a work in progress. Features I want to add are:

• The toggle hides when not in double page mode.
• The pages switch instantly when clicking the toggle.

This closes the issue I made #416

[rickymcmuffin]

Finished the changes. Let me know what you think!

[schroda]

looks good, just a few things

since the setting only effects the double page reader, it should only be shown in case it is selected

I think "offset first page" would describe the behaviour better (in code and text shown to the user)
the first time I read "invert double page" I didn't understand what it is supposed to do

not really relevant, but I would leave the "reader type" as the last setting, I think it looks better this way

[rickymcmuffin]

I made all the changes.

[schroda]

nice job improving the double page reader 👍

looks like there is also some issue with viewing a not downloaded/cached chapter with the double page reader, if you're interested in taking a look at it