[!!!][TASK] Streamline ElementBrowser mode handling

[!!!][TASK] Streamline ElementBrowser mode handling

The ElementBrowser now has a main entry point which takes
care of dispatching to the the various modes.
The main_*() methods of ElementBrowser are therefore made protected.

The $mode member of ElementBrowserController is not needed
by outside code and is therefore protected as well.

Additionally, the $content member of ElementBrowserController and
the printContent() method are removed.

Resolves: #69315
Releases: master
Change-Id: Ib9790915e72d3a5a460211a93ceeda24ee165c3e
Reviewed-on: http://review.typo3.org/42875
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>