frontend: Add template for Qt's invokeMethod

[cg2121]

Description

This adds a template for when OBS signals are called, so we don't have to call invokeMethod each time. This only works if the invokeMethod doesn't have calldata_t used with it.

Motivation and Context

Wanted to find a way so we didn't have to write a invokeMethod function for each OBS signal.

This is only used in MediaControls at this time. If this is accepted, I'll PR for other places later.

How Has This Been Tested?

Used media controls to make sure everything works as expected.

Types of changes

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

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.