Reduce needed handles by putting expand / collapse images in an image list

[joachimmarder]

A user that uses many Virtual TreeView controls reported that the count of used handles is quite high. The handle count can be reduced by putting the bitmaps FPlusBM, FMinusBM, FHotPlusBM, FHotMinusBM, FSelectedHotPlusBM, FSelectedHotMinusBM into an image-list, which uses only one handle. The same is already done with the check-images, which are contained in am TImageList.