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.