Drag/drop integrated with undo

[mikfire]

The undo functionality didn't work when dropping ingredients from the trees onto the recipe. As this is my primary method of creating recipes, the undo not work was frustrating. I fixed it.

I probably did some bad things by not creating two classes, but I was afraid of creating two different undo queues. This wastes some pointers, but makes sure there is one queue.

This is my second try at this, because git and I are not friends.

[matty0ung]

It's good you're fixing this. I remembered I hadn't made everything undoable, but I hadn't done a systematic search.

There is potentially a more elegant approach. Because Qt already allows undo/redo objects (QUndoCommand) to be grouped together, you could create, say, UndoableAddOrRemoveMultiple that just groups regular UndoableAddOrRemove objects together, in a similar, but more generic, way as is done in MainWindow::droppedRecipeEquipment().

[matty0ung]

Just for fun, if you want to see what this would look like, check out https://github.com/Brewken/brewken/pull/9 which I knocked up this morning. I haven't pushed the change back to Brewtarget, as it doesn't add any functionality, just a different way of doing the same thing but with Qt doing a bit more of the heavy lifting.

[mikfire]

I had toyed with a very similar approach when I realized I couldn't just overload a few things. I was uncertain of the side effects.

Given that the list form is a fairly isolated case -- it is really only needed in four places -- I rather prefer the separate classes. I might take it up later this evening if I am feeling motivated.