Add new option to support icon-view large thumb size (over 256x256 px)

If you use large screen resolution, as WQHD or 4K monitor, you can now use until 512x512 px size for thumbnails.
When this option is turned on/off from setings panel, user need to restart digiKam to take effect, and thumbnails
need to be rebuild over collection through MAntenance tool, to update digiKam thumbs database.
BUGS: 328430
FIXED-IN : 4.1.0