Feature: #97922 - Improve performance and usability while editing sys_filemounts

See forge#97922

Description

The two fields base and path of the sys_filemounts table are now combined in one new field identifier. The field contains the so called combined identifier in the format base:path, where "base" is the storage ID and "path" the path to the folder, e.g. 1:/user_upload.

An upgrade wizard is in place, migrating the two fields of existing records into the new field.

The TCA type folder is used in the backend form to select the entry point.

Impact

Editing sys_filemounts records in the backend is improved. Instead of selecting the storage first, reloading the form and selecting the entry point in a possibly large list afterwards, are users now able to select the entry point using the folder browser in a single step. This additionally improves the performance of the backend form, especially for storages with a huge amount of folders.