[TASK] sys_file/sys_file_reference override strategy

Until now, the sys_file_reference records use the regular "null value"
checkbox to switch between the value from sys_file and a custom value
the user can supply. This is however confusing for users, as there is no
explanation what the checkbox really does and also the placeholder text
is not removed when checking the checkbox (but an empty string is stored
in the database and thus used for output afterwards).

This commit changes the appearance to have a checkbox on top and toggle
between a classical read-only field and a real input field.

Change-Id: I2370b17c2cdf475aa6742cbca7cd12b6c8558130
Resolves: #49685
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/21931
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter