[feature] added draw background checkbox in composer items
(this allow to workaround the non-opaque background bug with atlas and is more consistent with the frame settings) )
fix for composer map item's background when non-opaque
upgraded to collapsible groupboxes
Merge pull request #408 from olivierdalang/showBackgroundCheckboxForComposerItem

[feature] added draw background checkbox in composer items
Fix #6389