UI: SubSynth: Fix getting stuck in oscilloscope

[AnClark]

In SubFX, once I switch to oscilloscope, I'll not be able to go back to
"Global" panel, as the envelope won't show anymore.
This patch should fix this problem: when click the "Global" tab button,
Zest should re-open the envelope view.

Reference: ZynPadSynth.qml (in src/mruby-zest/example)

NOTE: Maybe f21c3da should be dropped.

[fundamental]

Thanks for the fix.

I'm not sure how this would resolve the issue in add synth though. I'm fine reverting the add synth commit and replacing it with the same change presented here, though I'm not sure that the behavior of claiming the oscilloscope is a visualization rather than a subview is correct, nor am I sure about it being correct that the tabs should get disabled when moving into the oscilloscope view. If there's anything behavior wise wrong with my commit let me know and I'll revert f21c3da as proposed.

[AnClark]

I'm not sure how this would resolve the issue in add synth though. I'm fine reverting the add synth commit and replacing it with the same change presented here

Add synth has no problem when I feedback. I only found bug in Sub synth.