UI: Force fixed font in plain text edits

[cg2121]

Description

Since the Yami QSS changes the default font for widgets, we need to force the system fixed font for plain text edits.

Motivation and Context

Fix Yami bugs

How Has This Been Tested?

Opened log viewer to make sure it was the right font

Types of changes

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

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.
• [x] 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.

[WizardCM]

I considered this solution, but it'd also be required for any monospaced element, such as the CSS input field for browser sources.

[cg2121]

Updated to include other QPlainTextEdits.

The only way I could get this working is to set the stylesheet for each text edit. I couldn't get it working at all when setting a global stylesheet in obs-app.cpp

[cg2121]

Updated to just sub-class QPlainTextEdit. I feel like this is the least horrible way of accomplishing this.

[Hellscaped]

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