Insert Plugin before the marked one

hi.

took awhile but finally made it to qtractor >= v1.6.1.15git.fa4ce6

please test && tell
cheers

Hi Rui and @bluebell, I've found some inconsistencies in this new feature.

1. Plugins and pseudo-plugins behave differently.

If you don't select any, plugins go to the bottom of the list, while pseudo-plugins are placed at the top. This is especially inconsistent since pseudo-plugins are usually sends, and a send should always be placed at the end. Selecting an instrument and then adding a send that is placed before the instrument makes no sense.

2. The expected behavior by default when adding an item to a list is to place it at the after, not the before.

I know this means you can never place a plugin at the top if one is already there. However, it's much more predictable for the user to have it appear as the second item (which is easily accessible, unlike olders versions when it ended up last and could be hidden) and then drag it to the top, rather than sometimes ending up at the end and other times at the beginning arbitrarily.

Also, the bus list now always adds a new bus after the selected one.

Therefore, I believe that, in general, this would be a more predictable and comfortable experience for the user:

If no plugin is selected, the add-on ends up at the end of the list; otherwise, it ends after the selected one.

What do you think?