Fix a bug when files doubled when browsing a folder

[SergeiVorobyov]

• Fixed issue https://github.com/sylikc/jpegview/issues/147 when files appear twice in folder if set to be processed by WIC in config + supported by internal decoder library
• Code refactoring to simplify adding new file extensions to be supported.

Extracted this fix from https://github.com/sylikc/jpegview/pull/178 in order to have PRs more granular.

[sylikc]

I'll review this implementation also a bit later. Last I was looking at this I was thinking about whether this is cleaner than the old static list implementation... It feels similar, but I need to read through the code.

I didn't think it was clean before, but I haven't entirely read through this code yet. It looks cleaner though