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.
