Fix workflows run from the command line not exiting upon completion

[PathogenDavid]

(Submitted as a draft since I want to double-check some things tomorrow morning and probably revisit the wording in the explanation comment.)

This issue was actually two different issues in a trench coat:

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For a completely empty workflow (and perhaps other simple workflows?) the finally action will execute on the main thread, which causes Application.Exit to be called before Application.Run which means it doesn't exit.

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The second scenario is a workflow that end naturally and has a visualizer enabled. (Example workflow)

In this case the finally action will execute on a background thread. Application.Exit invokes FormClosing and FormClosed event handlers directly. This means those handlers are invoked off the main thread, which upsets many FormClosed handlers within Bonsai.

As an added treat, any exceptions thrown from finally actions are swallowed unceremoniously. As a result, one of the many unhappy FormClosed handlers interrupts Application.Exit and the signal to exit the message pump never reaches the main thread.

As a helpful side-effect, this means those exceptions are no longer ignored.

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I also fixed the NotifyIcon getting leaked (which was the underlying clause of it hanging around when the application exited--which is what I assume the old explicit hide was trying to prevent.)

As an aside, I noticed Observable.Finally is used all over within Bonsai to run dispose methods and such. It seems like it'd be a good idea to investigate that further and see if it's expected behavior from Rx.NET or maybe a bug.

Fixes #1740

[glopesdev]

@PathogenDavid great investigation, both of these are very significant regressions. Indeed when a workflow is executed in the editor all disposal events are marshalled correctly to the main window, but I failed to account for the asynchronous dispose in this case, interesting how this was able to fly under the radar for so long!

I will wait for the full PR, but just two quick comments:

• Do we need to worry about disposing the synchronizationContext itself, i.e. should there be a using statement there too? Maybe not in the case of the regular application path, but I'm thinking here about the case where someone calls Run programmatically.
• Re. the use of Observable.Finally, I would of course be very interested to audit the caveats of this pattern. Feel free to open a new issue for this if you find anything.

[PathogenDavid]

Good call on disposing the sync context.

I pushed better wording for my comment and that fix.

I got lost in the weeds a bit looking into the Observable.Finally thing and ended up filing an issue here: #1756 Maybe since you know Rx.NET better than I do something stands out to you there? It looks like it might become more of a problem when we upgrade to Rx.NET 6.0.0, so it might be worth fully investigating for Bonsai 3.0