Add functions to generate properties from frontend api

[cg2121]

Description

Adds functions to the frontend api to generate a properties widget. This is useful so frontend plugins no longer have to link to UI files.

Motivation and Context

Got sick of dealing with cmake files in frontend plugins.

How Has This Been Tested?

Tested with script properties. Still need to be tested with Decklink and AJA plugins.

Types of changes

• New feature (non-breaking change which adds functionality)
• 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.

[tytan652]

I don't wish to see more "Qt" frontend-api and the property view becoming a "stable" API.

[norihiro]

Decklink has its own UI to output preview and program to decklink-output. AJA has its own UI to output preview and program to aja-output. Majority of the code was just copied from decklink-output-ui to aja-output-ui. So does NDI.
Instead of improving the frontend plugins, I think the better implementation would be having a unified code in UI to setup preview, program, and probably sources to any outputs.

[cg2121]

Closing, as this is just a band-aid around the problem. Like @norihiro said, we need a plugin agnostic UI for outputs.