frontend: API call obs_frontend_add_transition()

[xurei]

This is my first PR on the OBS project, I hope I'll do it as expected :-)

Description

This PR adds the obs_frontend_add_transition() frontend API call.
This function allows to add global transitions in the collection without showing the popups.

This change required two modifications :

• Extend OBSBasic::AddTransition() with two new arguments : const char *transitionName = nullptr, bool showProperties = true. The default values ensures retro-compatibility.
• Add obs_source_t *obs_frontend_add_transition() to the frontend API, with the related documentation

Motivation and Context

I needed this to make a backup system for transitions in my plugin.
I didn't find any reason why it wasn't there, so I assumed it was forgotten.
However, there might be a reason why it wasn't implemented in the first place.

How Has This Been Tested?

I have tested this on Linux with success, not on Windows or MacOS.

Given the changes made, I don't believe much more testing is required.

Types of changes

• New feature (non-breaking change which adds functionality)
• Tweak (non-breaking change to improve existing functionality)

• Documentation (a change to documentation pages)

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.