Add state-saver support for PDF document

[Stefano Verzegnassi]

When a user closes the application, the application should save the current position of ListView/Flickable's ContentItem (a.k.a. "contentY"), in order to be able to restore the state the next time the same file is opened.

[Borim (borim)]

Add the possibility to add bookmarks would also be great. This way you can save the interesting positions of a document permanent.

[Siema (ziutek-hejho)]

Sometimes when I have serveral apps open when I got back to any app it just freezes and and after a monment unfreezes resetting to some default state: sms app goes from conversation with one person view to list of messages, phone app from last pohne calls to dial view and doc app from open document to list of documents. This is especially frustrating when reading several hundred-page document.

It would be really great to remember state for particular document and go back to that state. For me its the key feature for this app. On adroid I'm using Xodo, it's great.