QGIS Server - new project option imageQuality used for JPEG images

* JPEG image quality parameter in OWS / WMS capabilities of the Project properties dialog
* this parameter is only used for jpeg images
* a new parameter IMAGE_QUALITY can be used in the getMap request and will override the project value
Merge pull request #1403 from mdouchin/getmap_image_quality

QGIS Server - new project option imageQuality used for JPEG images