frontend: Fix TBar slider doesn't change when set via api.

[psyirius]

Description

Set the TBar value when it's not received from itself.
i.e: when TBar position is set via api: obs_frontend_set_tbar_position.

Motivation and Context

when TBar position is set via obs_frontend_set_tbar_position it calls OBSBasic::TBarChanged which is also the valueChanged signal handler of TBar.
Which in context of the signal handler shouln't set the value back.
but in context of calling externally (via api) we should set it.
so that the getter api obs_frontend_get_tbar_position which returns the value of tBar->value() will return a valid value.
Fixes: #11372

How Has This Been Tested?

set obs_frontend_set_tbar_position via plugin.
visually noticed the tbar moving.
return value verified using obs_frontend_get_tbar_position.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

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.