[BUGFIX] Context menu is not completely visible in filelist with scrollbar

If the foldertree in filelist is to long, the rightclick context menu on
the bottom of the window is not completly visible. With this patch the
position of the menu is calculated based on the windows width and height
and so completly visible.

Resolves: #82732
Releases: master, 8.7
Change-Id: Ied9204371491059efda61143e9ca9c2f3c8a9b9d
Reviewed-on: https://review.typo3.org/54390


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wittkiel Gruppe <ts@wittkiel-gruppe.com>
Tested-by: Wittkiel Gruppe <ts@wittkiel-gruppe.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>