Sync with what I did yesterday (without internet connection).

In libpalapelipattern:
* Allow to rescan the PatternTrader's configuration list.
* Finalize the libpalapelipattern API with the addition of the PatternPlugin class (which allows for multiple PatternConfiguration instances in one plugin).
* Randomize the random numbers.

In the pattern plugin subdirectory:
* Port the rectangular pattern to the new libpalapelipattern API.
* Add new pattern with hexagonal pieces (as a proof-of-concept implementation for arbitrary piece shapes).

In Palapeli itself:
* Add an autosaver class and adjust the settings interface to set autosave intervals. (This is not accessible from the interface by now.)
* Add a state manager class which records whether Palapeli's current stage is persistent. (This, and the above change, will make sense once I make a crash recovery interface.)
* Do not add relations to the Manager's list if there is a relation in the list that equals the given relation logically (by implementation of operator== for Palapeli::PieceRelation).

In general:* Remove some old TODO items.