The WaylandCursorTheme emits a signal whenever it reloads the theme.
This is used by CursorImage to clear the cache and reload decoration
and fallback cursor.
The theme doesn't load if we pass an incorrect size. This change
implements a resolution dependency size resolution inspired by the
one in Cursor::loadThemeSettings. Ideally this would be a size
different for each screen. As we don't have support for this yet, we
go for first screen.
