UI: Replace themeID and style hints with class property

[Warchamp7]

Description

In the past we've used a custom property called themeID to apply special styling to widgets. These widgets could be targeted using attribute style CSS selectors like
QWidget[themeID="addIcon"]. This approach was flawed in instances where we wanted to apply multiple of these adjustments on a single widget.

To solve that, we adjusted to instead using a boolean value which could be targeted with QWidget[addIcon="true"]

However, we have recently discovered that QSS has (almost completely undocumented) special handling for when widgets have a custom property named class, allowing the class selector like CSS

myWidget->setProperty("class", "addIcon toolButton")

QSS

.addIcon {
    /* styles */
}

.toolButton {
    /* styles */
}

Motivation and Context

Standardizes and unifies our approach for applying special appearance changes to widgets.

How Has This Been Tested?

Ensured that the appearance of specific widgets all looks correct

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)

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.
• I have included updates to all appropriate documentation.