Add more Lua functions for TAStudio markers

[YoshiRulz]

Based on #4272 by @c7fab (thank you, and please let us know if this is insufficient for your use case).

[c7fab]

At first glance it looks those functions can do everything that was possible with my PR.

[SuuperW]

I think it would be good to handle the case where the specified branch doesn't exist. So that tastudio.getmarker(0, 500) doesn't throw an exception. Maybe print a message saying no such branch exists, or maybe just return nil like when there is no marker on the given frame.

[SuuperW]

Actually, branches have an Id that can be obtained with tastudio.getbranches and this is used instead of an index in tastudio.getbranchinput. This seems like it could be useful as branch indexes can change by deleting one. Should this new function use the Id too?

Existing function tastudio.setbranchtext uses an index, not an id. So it's already not consistent in the API.

[vadosnaprimer]

Please provide a summary for dummmies, I don't have the capacity to remember how this is different from all the previous variants and I can't review it by looking at the technical side.

[YoshiRulz]

diff between force push
current API proposal
I implemented Suuper's suggestions of taking branch UUIDs as params and throwing less.
And to encourage people to use UUIDs more, I've also added get_branch_index_by_id so they can be used with existing index-based functions.

[SuuperW]

throwing less

I don't really like the idea of using the current marker set when the given branch ID doesn't exist.

I also think that the documentation given to users should specify what happens when the input is invalid.