frontend: Add copy paste functions

[exeldro]

Description

Add frontend copy paste functions

void obs_frontend_copy_sceneitem(obs_sceneitem_t *item);
bool obs_frontend_can_paste_sceneitem(bool duplicate);
void obs_frontend_paste_sceneitem(obs_scene_t *scene, bool duplicate);

Motivation and Context

Allow users from the vertical plugin to copy and paste sources from and to the main canvas

How Has This Been Tested?

On windows with changes to the vertical plugin

Types of changes

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

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.

[PatTheMav]

I'll have to give this a test locally but at first glance this seems fine and I also agree that it's preferable for 3rd party code to use these built-in functions to duplicate or alias existing sources.

[PatTheMav]

Tested this locally and seemed to work just fine.

[exeldro]

Switched methods to camelCase as requested

[exeldro]

Rebased and fixed conflicts