Add ability to export meshes for intersecting sources (synapses)

[igorpisarev]

Adds new Export button under the Meshes tab in the menu of an intersecting source.

After selecting the output directory, the meshes will be stored in a file synapses{segment_ids}.obj (or .bin), so that the filename will include the current selection of segments that generate the synapse meshes of the intersecting source.

[igorpisarev]

• Fixed filename too long error when trying to export synapse meshes for a neuron with a large number of merges. Instead of putting the full fragment ID list in the filename, we now use the segment ID list.
• A few other minor fixes related to mesh exporter dialog.