frontend: Update invokeMethod to use functions

[Warchamp7]

Description

Updates all frontend uses of invokeMethod with function pointers or lambdas.

Important

This PR is blocked until we drop support for Ubuntu 24.04 as this style of calling invokeMethod is not supported on Qt 6.5

Motivation and Context

Makes it easier to update or refactor code without having to worry about string references to slot functions.

How Has This Been Tested?

Compiled OBS and performed a few basic tests. I have not tested thoroughly yet.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.