The QImageControl and QImageLabel are forced to have a limit of 255 for file name length.

* It fixes issues #92 and #152
* The example of QImageLabel is changed to use CacheFolder property.
* The exception is throwed if the CacheFolder is not specified and the resulting file name length exceeds the 255 limit.
Merge pull request #221 from olegabr/bug-152

The QImageControl and QImageLabel are forced to have a limit of 255 for ...