[BUGFIX] Remove ElementBrowser::isReadOnlyFolder

ElementBrowser::isReadOnlyFolder is not required any more because the
check if the folder is writable has been moved to the methods that
create the file upload and folder creation forms.

The method and the parts where it was used were removed.

Additionally the check if the user is allowed to create folders
by TSConfig was moved to the createFolder method to reduce the amount
of duplicate code.

Resolves: #47648
Releases: 6.2, 6.1, 6.0
Change-Id: Ic6504c8def80012cbe420fc83539cfa859a53c0d
[BUGFIX] ArrayIterator::seek() warning in ElementBrowser

ElementBrowser calls Folder::getFiles() with wrong parameters.
Properly implement the file extensions filter.

Resolves: #51752
Releases: 6.2, 6.1, 6.0
Change-Id: I56468c79225e2d3baa5e5784571074532e2287ad
[BUGFIX] No double htmlspecialchars for filemount select

Since the labels and values of select items are run through
htmlspecialchars by the FormEngine there is no need to use
htmlspecialchars in the renderTceformsSelectDropdown() method which
generates the select items for the filemount Backend form.

The current code will htmlencode the select value twice which results
in a htmlencoded value in the database which causes problems with
directory names that contain special characters.

Resolves: #54027
Releases: 6.2, 6.1, 6.0
Change-Id: I7ec8262f6c3d20879cde0679636a6a8e5c1d19cd