Instrument drop-zone indicator

Nobody has claimed this yet.

**Edited 2014-12-31 @tresf, added comments from @DeRobyJ's bug report, mockup of drag-zones**

> If you replace the instrument of one track by drag-n-dropping a plugin/preset onto that track, you've lost the old instument.
> Of course undo-redo will make this less harmful, but it would be great to have a pop-up saying like:
> "Are you sure you want to replace this instrument?
> The current one will be lost!"
>
> **Actions:**
> - Yes - Do it like normal
> - No - Add a track with the dropped instrument
> - (Cancel - Do nothing)
>
> [extra: if the old istrument of the track is the default preset, don't show the pop-up and do the "yes" action]



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The way an empty instrument from sidebar, can be added to a project, can be problematic.
Reproduce:
Take any project
duplicate a track until SE is completely full
take any instrument from the sidebar ad add it.

Intuitively, a user will 'add' this new empty instrument to the project _inside_ SE, but if that is done, the new instrument overwrites the existing preset that is closest to the mouse.
Old users know, that the trick is to add the instrument _outside_ SE -eg on the scroll-bar, but that is definitively not intuitive.
I sugest some kind of 'safety' pop-up:
_"Should this empty instrument *_overwrite*\* this **existing preset?** y/n"*
If the user says n, the new instrument is then added as last instrument in the project
what do you think-

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Start by reproducing the sidebar drag-and-drop behavior described in the issue, including dropping onto a full project and replacing an existing instrument. Review the attached mockups and the requested Yes, No, and Cancel outcomes. Done means accidental replacement is clearly confirmed or avoided, while declining adds the instrument as a new track.

Written by the indexing model from the issue text.