This commit changes the createButtonImage method in CTabFolder. Until now the requested image data were created once and are scaled to other zoom settings with an AutoScaleImageDataProvider, which is a destructive operation when scaling it to a smaller zoom. With this change the whole logic is moved into an ImageDataProvider, so each time ImageData for a specific zoom value is requested, it will be created from scratch for this zoom value.

Contributes to #62 and #131