Revert "workaround to adress badly attached thumbnail views (no more time to investigate on it...) + prevent overwriting scenes"

This reverts commit 0fd15f62.

workaround to adress badly attached thumbnail views (no more time to investigate on it...) + prevent overwriting scenes

This mainly changes document mode behaviour in two ways:
1) When deleting 2+ items, a new document was selected in the list. Now, the current document is selected,
or if it has been deleted, a the first document in the list is set as
current document.

2) When deleting the last item in the trash, no document was selected.
Now, the current document is selected instead.

When deleting a document that was in the trash, the following item was
selected but also set as current document. This is now fixed.

This fixes an issue where if one document was imported with a different
DPI than the current one, any document created thereafter would have
this same value (which could then cause problems if a PDF was added to
that new document).

Saving this value to UBDocumentProxy not only makes more sense, it also
fixes this issue.

This reverts commit 4aa0ee61.

(Not doing so resulted in either a trashed item being selected when
returning to board mode, or a crash if the item had been permanently
deleted)

The documents' metadata.rdf file is now persisted only when a scene in
the document is also persisted; as well as when the document is modified
(trashed / path changed) in the the Documents pane.

Code was cleaned-up a bit too (added a forgotten return value, etc)

(Reading is still done synchronously, for now at least)

During trashing of multiple documents, a new document was selected
every time that one was moved to the trash -- even if it was about
to be trashed itself. This is no longer the case; the
selectADocumentOnMultipleTrashing() method was added to select the
first unselected document found (but this is currently not used, as
it seems OK to just select no document when we have just deleted a
whole selection)

Also quick code clean-up in UBDocumentController

When both a folder and items it contained were selected, and trashed,
the documents could sometimes be deleted twice and thus, permanently
deleted when one just intended to move them to the trash.

When the time comes to merge dev into master (for real), the
following may be useful:
https://metlos.wordpress.com/2012/01/13/
git-merging-after-a-revert/

Q_WS_MACX -> Q_OS_OSX
Q_WS_MAC -> Q_OS_OSX
Q_WS_WIN -> Q_OS_WIN
Q_WS_X11 -> Q_OS_LINUX