WIP Support multiple open items

[mvasilak]

Adds support for multiple open items.

• Keep track of open items in order of most recently opened
• Show in items navigation bar button that restores last item and shows count of currently open items
• Save/restore state of open items
• Add support for PDF items
• Add support for note items
• Show in presented item navigation bar button menu that allows to quickly switch to another open item
• Allow user to change order of presented open items

In consideration

• Support separate open items for multiple windows
• Opening an already opened item in another window, should switch to that

[mvasilak]

@michalrentka this is ready for a first look. Things to keep in mind:

• I tried not to upset existing architecture a lot, hence at the time being I refrained from using something like child view controllers to switch between open items.
• Instant presentation solution using screenshot window might be a bit extreme. I'll investigate if it can be done in a more elegant way, e.g. using custom transitions.
• In practice cold switch between items seems adequate, but we may need to consider hot switch of view controllers kept in memory, if there are UX issues.

Next steps could be keeping separate open items for separate windows, support for notes or other items, customization of the menu, or new UI (e.g. tabs) that makes use of this facility.

[michalrentka]

Looks good to me so far 👍. Just one comment regarding the ReadItemGloballyDbRequest which I think is not necessary.

[mvasilak]

@michalrentka ready for another look. New additions:

• User order is maintained in the bar buttom menu (not editable at the time being, it is always the order of first time item is opened).
• Support for notes.

[mvasilak]

@michalrentka ready for another look. New additions:

• Validation of open items on app restoration.
• Observing for current open items deletion, in order to validate/update open items list.
• Support for different open items per different app scene.

Up next, UI improvements, initially using a popover instead of the simple menu for switching open items.

[mvasilak]

Due to many changes when rebasing, this PR is closed to use a new branch.