Update jQuery to v1.8.3

This was required due to to a bug in the version we used before. It came into
effect when exporting a neuron catalog SVG and raised an error.

I went through the list of removed and changed functions [1] and found only
smaller things. Since these are rather minor issues, I'll commit them
separately.

[1] http://blog.jquery.com/2012/08/09/jquery-1-8-released/
Remove unused jQuery.dd widget

This "msDropDown" UI element was added in a40a70f621 and is not used
anymore (the back then created CATMAID widget also doesn't exist
anymore).

Besides not being used, the widget in its current version is also not
compatible with jQuery 1.8.3, to which we just moved. The deprecated

  .data("events")

was removed and this was used by jquery.dd.
Move connector selection widget into widgets/ folder
Update jsTree to 2. fix release of v1.0-rc3

This was needed due to the jQuery update.
Merge branch 'update-jquery-to-1.8.3'