Nightly Build: MIDI stops when other track windows selected

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### Bug Summary
I have two tracks assigned to MIDI Channels 1 and 2. When I select track one which is midi channel 1 and I have my controller set to Midi Channel 2 the notes do not register and no sound is heard until I click the track window again.

#### Steps to reproduce
1. Open two tracks
2. Set each track to a separate MIDI channel
3. Set your controller to the channel for one of the tracks
4. click on the general settings windows for the other track
5. press a key on your midi controller

#### Expected behavior
The MIDI channel set on the controller should always play whatever track is set to that channel.
#### Actual behavior
The MIDI controller only plays the channel it is set to when no other General Settings window is selected.
#### Screenshot

#### Affected LMMS versions
Version 1.3.0-alpha.1.33+g4f74151f0 (win32/x86_64, Qt 5.9.8, GCC 7.3-win32 20180312).

#### Logs

Click to expand

No source file or test is named in the report. First reproduce the issue with two tracks on separate MIDI channels, then trace MIDI input handling when another track's General Settings window is selected; it is done when controller notes consistently play the track assigned to their MIDI channel.

Written by the indexing model from the issue text.