Only show files in the `tl_content.posterSRC` file picker

Limits the selection options to show only files in the preview image file picker.

Signed-off-by: xchs <github@gmx.net>
Merge branch 'hotfix/posterSRC-eval' of git://github.com/xchs/contao-core into hotfix/3.0.7
Back port the changes from #5560
Apply the access restrictions in the front end only (see #5603)
Correctly set the end date of duplicated events (see #5608)
Cherry-pick 'Fix the tooltip display in the repository module (see #5540)'
Purge the internal cache when disabling the Contao safe mode (see #5579)
Update the example website
Merge branch 'hotfix/3.0.7' into develop

Conflicts:
	contao/main.php
	system/docs/CHANGELOG.md
	system/modules/core/dca/tl_settings.php
	system/modules/repository/themes/default/backend.css
	templates/music_academy.sql
Update html5shiv to version 3.6.2