Counting: refactored handling of resizehandles
Counting: switch from @mainthreadonly to @pyqtSlot mechanism everywhere
Added the option to only show either 'load images' or 'load from stack' in AddFile buttons
reverting changes partly from cca599c77c56a643647e3eac94077dcd2b79146d
As this influences quite a bit, should only be done after the release
Revert "Added the option to only show either 'load images' or 'load from stack' in AddFile buttons"

This reverts commit 4881dd900ed8a9dccd2d6dad2c7aa1e2dbe1bc59.
OpThresholdTwoLevels: Force cache to be stored in xyzct order, since that is backwards-compatible with existing project files.
shell: Prevent all threadRouter events from handling anything after the app starts shutting down
(Fixes a segfault.)
DataSelectionGui: Use QMessageBox.critical() instead of QMessageBox.error(), which does not exist.
Fixes #851
Use late import statement to avoid importing sip in headless mode.
OpPixelClassification: For the label cache, choose a blockshape of about 1MB no matter what axes are present.
debug logging statements.
logging: Added more logging config entries.
tests: Fixed pixel classification serializer test.
Merge branch 'public_master' into release_master