Drag and Drop of Palette Elements

I have been playing with the drag and drop functions and would like some feedback for how users think they should behave. Referring to this request, there are some cases where "simply" inserting could be less than desirable. I'll use this image as an example.

Moving the double repeat element in the bottom right, and dropping it onto the first element in the top row.

Inserting will cause the empty cell at the end of the row to get pushed into the second row, and the last element in the second row to go into the third. If I'm organizing my palettes into rows, this will cause headaches.

Moving the double repeat element in the bottom right, and dropping it onto the first element in the second row.

Inserting will push the last element into the next row.

I could create it so it preserves rows where possible. In the first case, it would remove one empty space from the first row, and move the existing 4 elements one cell to the right. This would preserve the layout of the other 2 rows. In the second case, it would push the last element into a new row on its own, and the last row would get pushed into a 4th row. Before I make a pull request with my changes, I would just like to get feedback. "Simple" inserting can be found in my repo 773adc0

For now it only works with cells inside the same palette.

Also fixing the inconsistent highlighting while dragging.