add sub-method getImageExtensions() with EP "OOMEDIA_IMAGE_EXTENSIONS"
serve non-static imageExtensions from OOMedia as var to scripts
use var imageExtensions in REX_MEDIA widgets..
rename JS var: imageExtensions -> rex_imageExtensions
move EP "OOMEDIA_IMAGE_EXTENSIONS" ..
use $REX['IMAGE_EXTENSIONS']
use $REX['MEDIAPOOL']['IMAGE_EXTENSIONS']
use $REX['MEDIAPOOL']['IMAGE_TYPES'] & $REX['MEDIAPOOL']['ALLOWED_DOCTYPES']
Merge branch 'refs/heads/core/expandable_imagetype_handling'

Conflicts:
	redaxo/include/master.inc.php