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 (only move first & last currently)

In consideration

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

Previously discussed in #782

[mvasilak]

@michalrentka @dstillman
this PR is ready for some more eyes and perhaps beta testing. It has the underlying facility to support:

• multiple open items
• switching between open items
• opening new items
• closing existing items
• moving open items to another position (user index)
• keeping track of last opened date for each item
• opening an item in its corresponding window/scene if it's already open

It currently supports PDF documents and notes, and should be straight-forward to add support for e.g. web snapshots and ePubs.

UI-wise, it includes a bare minimum implementation using simple UIMenus, with these features:

• In the items screen, a bar button appears with a number indicating the number of open items.

• Tapping the button, will open the last opened item.
• Tapping and holding will show a menu that allows opening specific item, or closing all open items.

• Similar bar button appears in the PDF document/note screen.

• Tapping the button will always show a menu. It contains all the open items as before, but also adds a sub-menu for the current item. Said sub-menu allows to close current item, move to start (if not already), move to end (if not already), close other items (if other items are open).

Next (possible) steps (in separate PRs) are (a) an actual tab bar and (b) instead of using UIMenus create a custom view that allows e.g. closing any item and re-ordering.

Feedback regarding icons and copy used will be much appreciated.

[michalrentka]

Code-wise there are just some minor mostly style-related adjustments. After TO-DOs are resolved this looks good. I'll spend some more time testing this on device, but to me it looks good for at least internal user testing.

[michalrentka]

Some bugs and smaller issues I caught:

1. On iPhone, the square bar button item for tabs seems to have a lot of space around it
   
2. When I open multiple PDFs, then I scroll to some page on one, switch to another one and switch back to the original one, the original one is not on the page I scrolled to, but on page that it was when I opened it

[mvasilak]

@michalrentka

1. On iPhone, the square bar button item for tabs seems to have a lot of space around it

It's the standard spacing for bar button items created with an image, it just seems like that because is next to another such button. We can use a custom view button instead, to decrease the image insets. Maybe we should consider using UIBarButtonItemGroup to group related buttons when there are space constraints.

2. When I open multiple PDFs, then I scroll to some page on one, switch to another one and switch back to the original one, the original one is not on the page I scrolled to, but on page that it was when I opened it

Good catch, fixed in 33b5f63

[michalrentka]

Looks good to me now, I didn't find new bugs yet. We should probably make an internal build for others to test or push it to beta. There are a ton of changes, so we should definitely test this thoroughly. @dstillman can decide.

[mvasilak]

@michalrentka rebased to master, to include the activity/scene title logic, and made some relevant improvements for the note editor.