This is a common practice in C projects, which simply enforces that
each header must compile cleanly without implicit dependencies on
other headers (see also the previous commit).
This patch fixes the issue that window icons become the fallback
one after Reconfigure, by setting scaled_icon_buffer->view_app_id (and
other properties) in scaled_icon_buffer_set_view().
The reason why the window icons were displayed before Reconfigure is that
applications usually enable decorations and then set app_id which fires
handle_view_set_icon().
This patch also changes the semantics of scaled_icon_buffer: rather than
calling scaled_icon_buffer_set_app_id() every time an app_id is set, we
can now call scaled_icon_buffer_set_view() just once so that multiple
scaled_icon_buffers bound to a window are automatically updated when an
app_id is set or new icon is set via xdg-toplevel-icon-v1.
This commit adds <theme><fallbackAppIcon> that configures the icon name
to be used when lookups for an application icon is failed. Its default
value is set as 'labwc' which shows our labwc logo.
