Update VERSION comment with instructions

Instructions about separate source and Debian package versioning.
Removed of libsokoengineqt package.

This library exists solely to be able to link tests against it. Otherwise,
it would be integral part of bosp_app. Instead of creating shared,
installable library, we build it as static library and link BoSp app and
tests with it. Library itself is never installed.
Cleaning up
Merge branch 'feature/reorganize_debian_package' into development

Removed libsokoengineqt package. This library existed solely to be
able to link test suite against it. It is now built as static library
and both, BoSp app and tests are linked against it. Package for that
library is no longer needed.
Added GameBoard::resize_and_center
PuzzleEditor now works on copy of GameBoard.
Created GameBoardEditor and delegated GameBoard editing from PuzzleEditor.
Removed metadata editing from PuzzleEditor

This complicates implementation of undo stack. It would be nice to have it
here, maybe it can be returned at later stage.
Removed model changes tracking from PuzzleEditor.
Removed local QToolbar from PuzzleEditor
More icons
Interface for document workflow in PuzzleEditor.
PuzzleEditor workflow in MainWindow
Moved Undo/Redo actions out of MainWindow
Moved MainWidow to bosp_app
Discarding puzzle editor changes on collection close.

If puzzle editor wasn't visible all the time, we wouldn't have to do it
this way. On the other hand, it is dangerous to leave puzzle open in
editor one collection was closed.
Added GameBoard::is_playable
Added PuzzleBrowser::first_selected_puzzle
Revision of PuzzleEditor document workflow interface.
Implemented PuzzleEditor document workflow interface.