DataSelection: Preliminary UI support for selecting and loading a DVID volume (doesn't save to a project file...)
DataSelection: Fixes to handle urls.
DataSelection: Add dvid volumes as urls, not temp files.
DataSelection: Support url edgecase.
DataSelection: dvid: Hostname is now provided by the contents browser ui.
Applet GUIs fixes:
- In stopAndCleanup(), don't touch QWidgets that might have been deleted already.
- When handling a ThunkEvent, check to make sure the object is still alive.
fixes #772
startShellGui: Set Qt.AA_X11InitThreads on Debian also.
tests: Assume recorded tests are executed from within the tests dir.
tests: counting: Relaxed an assertion in the GUI tests.  Seems to be necessary on some linux platforms, even though it passed on mac.
tests: counting: Relaxed an assertion constraint.
FeatureSelection: Don't access any gui items in stopAndCleanup()
Fixes #777
shell: Actually quit if the user says Quit and then "Save".
applets.base: Don't access temporary drawers that have been deleted already.
Fixes #778
ilastik.py: When recording or playing back, disable 3D.  Also, use the same QApplication type in both cases.
LayerViewerGui: Pass parent to VolumeEditor, as required by recent volumina change.
testing: Improvements to event recording system.
(Note that the entire recording system, including these changes, will soon be extracted into a separate library.)
testing: Removed recording code, which now resides in a separate repo (a new dependency)
eventcapture dependency is now optional unless you actually try to record/playback.
Added submodules dir and added eventcapture repo to it.
Updated eventcapture submodule.