Replace QRegExp usage for better Qt6 compatibility

[RytoEX]

Description

Replace QRegExp usage for better Qt6 compatibility.

Motivation and Context

The existing QRegExp usages are just doing basic string matching to remove filename extensions. QRegExp was removed in Qt6, so let's just use QString for this functionality instead.

How Has This Been Tested?

I don't have any VSTs installed on my system, so I haven't tested this. I'll need someone else to make sure that this change is acceptable.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [ ] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.

[RytoEX]

It may be worth noting that the original functionality might have been case-sensitive, while the code I wrote here is case-insensitive. I don't know what is preferable.