#284 Drag and drop area detection

[HarbourHeading]

Fixed issue #284. I'm sure there is a better way to calculate the offset based on the selected item's image (preview) size, but this is an OK solution as item dragging is rarely done at the same time as other heavy computations, and only calculated once.

Note: Only tested on inventory_grid_stacked_transfer and with all unittests.

[peter-kish]

Hmm.. am I missing something or this doesn't really change anything in the drag and drop behavior (see gif)?

[HarbourHeading]

First 15 seconds is the current version of gloot, then the last 15 seconds is my version. It centers the item visually.

Edit: Though yes, it does not necessarily "fix" the drag and drop, but it makes it at least a little more visually responsive.

[peter-kish]

Ah, I see. But that is not quite the behavior I was going for. I would like to avoid making the items "jump" to the mouse coordinates (had complaints about it in the past).

Instead, I did some debugging myself and came up with a fix: #300
Not the prettiest solution, but since it seems to be working I'll leave the refactoring for another day.