Improve movement of symbols within palettes: use "insert" instead of "swap"

When you move a symbol to another part of the palette the program simply swaps two symbols.

If for example, in the advanced line palette , you were to drag the ambitus on to the 1st time volta, you would then have to drag the volta back into position. This would cause the 2nd time volta to move out of position. Then you have to drag that back into position and so on …

Much better way: When the user drags and drops a symbol from inside a palette on to another one (in the same palette), the symbol is simply inserted and the other symbol pushed down the order. No extra work required.

For symbols dragged in to the palette (from the score page) it may be a good idea to keep the current arrangement: they are added to the end of the list. The user can then decide where to position them.