This allows encapsulation of properties relating to the context
of a search, such as a target map extent. Locator filters could
use this to prioritise results close to the current canvas extent.
This adds a new "locator" bar to the QGIS status bar. If you're not
familiar with QtCreator's locator, it's a quick search bar
(activated by Ctrl+K) which displays matching search results
from any number of registered search filters.
Search filters are subclassed from QgsLocatorFilter, and
added to the app's locator via iface.registerLocatorFilter(...)
Searching is handled using threads, so that results always
become available as quickly as possible, regardless of whether
any slow search filters may be installed. They also appear
as soon as each result is encountered by each filter, which means
that e.g. a file search filter will show results one by one
as the file tree is scanned. This ensures that the UI is always
responsive even if a very slow search filter is present (e.g.
one which uses an online service).
This framework is designed to be extended by plugins, such as
OSM nominatim searches, direct database searching (i.e. Discovery
plugin), layer catalog searches, etc...
