[CLEANUP] main_rte function of the ElementBrowser

It splits 450 lines function into smaller pieces
and removes some duplicated code in the process.
It also opens a way to further duplicate removal
e.g. the code for selecting current folder.
This patch is not changing functionality,
just moving code around.

* splits main_rte into smaller chunks
* make main_rte method signature be consistent
  between class implementations and usage
* removes ~100 lines of duplicated code
  getPageSelectorHtml and getFileSelectorHtml
  are now inherited in the BrowseLinks
* removed few "@todo Define visibility" for methods
  which are called from outside

Resolves: #56781
Releases: 6.3
Change-Id: I7679017b1e215ea2d4da46fa30314313bc9980c5
Reviewed-on: https://review.typo3.org/28269
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring