implement the panelLayout_callback
the callback is now defined for a certain panelLayout-key
the callback is now named "panel_callback"
fixed typos
enable assigning fields to different filter panels
updated changelog with the two new features (panel_callback, filter assignment)
Optimize filterPanel callback to follow Contao Core
cs
cs
fixed wrong reference in panels variable
Merge branch 'panelLayout-extended' of git://github.com/Toflar/contao-core-fork into feature/extended-panel-layout

Conflicts:
	system/docs/CHANGELOG.md
Minor code formatting corrections
Add a CHANGELOG entry before merging
Merge branch 'feature/extended-panel-layout' into develop