frontend: Fix deprecated header order in Add Source dialog

[jcm93]

Description

Fixes the placement of the "deprecated" header in the Add Source dialog such that all deprecated sources actually lie beneath it.

Motivation and Context

Some deprecated sources would lie above the "deprecated" header because the iterator that moved the sources removed items from the list while iterating through it (skipping some items).

This PR just makes the iterator operate in reverse order to avoid skipping any items as they're removed.

How Has This Been Tested?

Tested on macOS to verify that deprecated sources all now sit beneath the header. Verified that a deprecated source at the first or last position in the list is scooped up by the iterator.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.