build: Move GcalSourceDialog to gui/calendar-management

Soon we will start populating this subfolder with many other
calendar management files, and to avoid making the gui/ folder
a mess, move source dialog to it.

src/meson.build

@@ -76,6 +76,7 @@ calendar_incs = [
   top_inc,
   include_directories('core'),
   include_directories('gui'),
+  include_directories(join_paths('gui', 'calendar-management')),
   include_directories('search'),
   include_directories('utils'),
   include_directories('views'),
@@ -101,6 +102,7 @@ sources = files(
   'core/gcal-shell-search-provider.c',
   'core/gcal-timer.c',
   'core/gcal-time-zone-monitor.c',
+  'gui/calendar-management/gcal-source-dialog.c',
   'gui/gcal-application.c',
   'gui/gcal-calendar-popover.c',
   'gui/gcal-date-chooser.c',
@@ -110,7 +112,6 @@ sources = files(
   'gui/gcal-event-widget.c',
   'gui/gcal-multi-choice.c',
   'gui/gcal-quick-add-popover.c',
-  'gui/gcal-source-dialog.c',
   'gui/gcal-time-selector.c',
   'gui/gcal-window.c',
   'search/gcal-search-button.c',