project: Split meson files in subdirs

It allows us to better control what each subdir does, etc. This is
only the first step; we want to also split GResources, etc.

src/gui/calendar-management/meson.build

@@ -0,0 +1,9 @@
+calendar_incs +=  include_directories('.')
+
+sources += files(
+  'gcal-calendar-management-dialog.c',
+  'gcal-calendar-management-page.c',
+  'gcal-calendars-page.c',
+  'gcal-edit-calendar-page.c',
+  'gcal-new-calendar-page.c',
+)

src/gui/meson.build

@@ -0,0 +1,18 @@
+subdir('calendar-management')
+
+calendar_incs +=  include_directories('.')
+
+sources += files(
+  'gcal-alarm-row.c',
+  'gcal-application.c',
+  'gcal-calendar-popover.c',
+  'gcal-date-chooser.c',
+  'gcal-date-chooser-day.c',
+  'gcal-date-selector.c',
+  'gcal-edit-dialog.c',
+  'gcal-event-widget.c',
+  'gcal-multi-choice.c',
+  'gcal-quick-add-popover.c',
+  'gcal-time-selector.c',
+  'gcal-window.c',
+)