Some filters are fast enough to return results that it's overkill
to run them in a background thread - add a flag to these filters
to allow them to run (blocking) in the main thread instead.
- add a clone() method to filters, and always search using the
clone instead of the original filter
- add a prepare() method to filters, which is always run in the
main thread and can be used to prepare the filter for safe
background execution (e.g. creating feature iterators in advance)
- don't use QtConcurrent to perform searches in background threads,
since it is not safe to use with QObjects
- instead manually create threads and ensure that cloned objects
are always moved to the thread that they will run in, to ensure
that they correctly have thread affinity with the thread in which
they are executed
required connections to a QgsLocator
Use this QgsLocatorModel subclass when you want the connections
between a QgsLocator and the model to be automatically created
for you. If more flexibility in model behavior is required,
use the base QgsLocatorModel class instead and setup the
connections manually.
