Changing time signature causes incorrect BB track to be displayed in the BBEditor

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### Bug Summary
When time signature isn't n/n, BBEditor displays incorrect tracks. I tracked the bug to the TimePos::getBar() function call, which gives inaccurate results. Unfortunately fixing this problem will require some class restructuring, and given I am new to the project, I wasn't able to do so.

#### Steps to reproduce
Create multiple BB tracks, enter steps for clarity, change time signature.

#### Expected behavior
I would hope correct track would be displayed, which is not the case.

#### Actual behavior
Incorrect track is displayed. This is because the track number is calculated as m_ticks divided by m_ticksPerBar, where m_ticks doesn't get updated as time signature changes. That means that instead of track 0, track 1 is displayed when the time signature is 5/4 (because 192/240 is 0), etc..

#### Affected LMMS versions
Both stable and nightly.

#### Logs

Click to expand

Start by tracing the BBEditor track-selection path through TimePos::getBar(), especially how m_ticks and m_ticksPerBar change after a time-signature change. Reproduce the issue with multiple BB tracks and a non-n/n signature such as 5/4. Done means the correct track remains displayed after changing the time signature.

Written by the indexing model from the issue text.