HaikuDepot: Improve Common Operations around STL

[apl-haiku]

A while ago there was a decision made to switch the internal collections classes from a custom list class to use STL. The rationale was around needing more algorithms and collection types that would have been otherwise difficult to achieve.

In the interim it has been observed that some of the code is harder to read because of this and also that some use of the STL has been erroneous which had lead to memory issues.

Stippi has suggested adding a ListUtils (name?) class with some static methods to help provide common operations such as Remove(stlList, item) so that the code is easier to read and common pitfalls can be avoided.