image-source: allow upper case image extensions in image and slideshow

[reald]

Description

Allow upper case image extensions in image and slideshow.

Motivation and Context

Before obs only accepted lower case image extensions (*.bmp, .jpeg, ...) in image and slideshow plugin. Now upper case (.BMP, *.JPEG, ...) are allowed, too. This is necessary at least on linux file systems.

How Has This Been Tested?

Compiled, tested functionality on: Ubuntu 18.04.4 LTS (bionic)
Kernel: Linux blank 4.15.0-91-generic #92-Ubuntu SMP Fri Feb 28 11:09:48 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

Types of changes

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

Checklist:

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

[kkartaltepe]

This is an upstream bug present in Gtk native dialogs. It does not happen in qt dialogs (could not replicate on head with native qt dialogs).

https://bugreports.qt.io/browse/QTBUG-51712

I dont think this is too important either as we may end up removing all native dialogs which would resolve this issue for gtk users.

[jp9000]

If we have to add this for this string, we'll have to add it for every single string where we have file extensions in the entire program.

Maybe someone can make it use qt dialogs or something. I'm not merging this, I'd rather make everyone rename every single extension than to merge this because I'd have to do this for every single instance where we need file extensions and quite frankly that makes me nauseous.