webserver: cosmetic refactoring
webserver: add support for If-Unmodified-Since HTTP header
webserver: add Cache-Control HTTP header to cacheable responses
webserver: handle Cache-Control: no-cache and Pragma: no-cache
webserver: use defines for HTTP header fields and values
webserver: silence warning
webserver: refactor IsAuthenticated()
[infomanager] added Container(id).CurrentItem
[mediawindow] remove unused function OnFinalizeFileItems
BaseRender: Allow 1px wrong in aspect ratio to not do completeley refit the image
[addons] display a more helpful message on python errors
Merge pull request #5894 from tamland/script-error-message

[addons] display a more helpful message on python script error
Merge pull request #5872 from mkortstiege/infomanager_currentitem

[infomanager] added Container(id).CurrentItem
[infomanager] adds container.art
[art] tvshow season art not set correctly when item list was flattened
[art] add container art to music paths
[skin] fix weather layout re-touched
[infomanager] nuke old container art infolabels
[gui] bump gui version to 5.4.0
Merge pull request #5911 from mkortstiege/nuke_finalize

[mediawindow] remove unused function OnFinalizeFileItems