search: Introduce a search engine

This is architectured in such a way that it will
allow us to introduce other searcheable terms in
the future.

All the functionality that in the past was in
GcalManager is now spread across GcalSearchModel,
GcalSearchHitEvent and GcalSearchEngine.

src/meson.build

@@ -21,6 +21,10 @@ gcal_deps = [
 
 sources = files(
   'search/gcal-search-button.c',
+  'search/gcal-search-engine.c',
+  'search/gcal-search-hit.c',
+  'search/gcal-search-hit-event.c',
+  'search/gcal-search-model.c',
   'utils/gcal-date-time-utils.c',
   'utils/gcal-thread-utils.c',
   'utils/gcal-utils.c',
@@ -51,7 +55,6 @@ sources = files(
   'gcal-night-light-monitor.c',
   'gcal-quick-add-popover.c',
   'gcal-recurrence.c',
-  'gcal-search-popover.c',
   'gcal-shell-search-provider.c',
   'gcal-source-dialog.c',
   'gcal-time-selector.c',