form option #allow breaks term order

Problem/Motivation

The form option #allow takes a list of terms that are used to filter the terms to be displayed.
But instead of just filtering the term tree, the terms get sorted according to the filter list.

BTW the responsible code seems to outdated. The $filter variable is treated as string, but the calling code treats it as boolean.
That leads into the wrong path of the if else statement. The code could be simplified.

Steps to reproduce

Proposed resolution

Keep the order of the tree and just filter it.

Remaining tasks

User interface changes

The term list keeps its order (bugfix)

API changes

none

Data model changes

none