Skyline: Improve tutorial screenshot preview UI

[brendanx67]

• added a standard ToolStrip toolbar with image buttons and keyboard shortcuts (with ability to switch back to text buttons)
• implemented ability to Pause during multi-screenshot progress
• moved progress bar to New Screenshot pane
• added button to switch to downloading Old Screenshot from the website
• persisted preview state run-to-run in a new TestUtilSettings user.config file
• improved error handling not to interfere with tests (not use MessageDlg)
• added MultiFormActivator class to improve co-activation of Skyline and ScreenshotPreviewForm (not yet used)
• implemented Nick's suggested changes from PR# 3153

The new UI looks like this when showing images from the website:

[brendanx67]

I have now used this new UI to review the screenshots for our first 4 complete tutorials (MethodEdit, MethodRefine, GroupedStudies, ExistingExperiments) and it works well for me. Eddie and Eduardo, interested in having you try it, as you will have different usage patterns and may run into issues still that I am not seeing.

[eduardo-proteinms]

@brendanx67 I have been testing it out today and some of the issues I was having before don't seem to occur anymore. 
It still looks like pressing continue too quickly can cause odd interactions. Do you think we should disable the continue button until the new screenshot is loaded? Here is some of the issues I have seen when continue is pressed too fast. 

Previous screenshot is treated as new screenshot

InvokeRequired returns False

pwiz.Skyline.Util.AssumptionException
   at pwiz.Skyline.Util.Assume.Fail(String error) in C:\Users\Eduardo\repos\pwiz\pwiz_tools\Skyline\Util\Util.cs:line 2041
   at pwiz.Skyline.Util.Assume.IsTrue(Boolean condition, String error) in C:\Users\Eduardo\repos\pwiz\pwiz_tools\Skyline\Util\Util.cs:line 1967
   at pwiz.SkylineTestUtil.ScreenshotManager.ActivateScreenshotForm(Control screenshotControl) in C:\Users\Eduardo\repos\pwiz\pwiz_tools\Skyline\TestUtil\ScreenshotManager.cs:line 254
   at pwiz.SkylineTestUtil.ScreenshotPreviewForm.TakeScreenshot(ScreenshotValues values) in C:\Users\Eduardo\repos\pwiz\pwiz_tools\Skyline\TestUtil\ScreenshotPreviewForm.cs:line 570
   at pwiz.SkylineTestUtil.ScreenshotPreviewForm.UpdateScreenshotsAsync(Boolean showWebImage) in C:\Users\Eduardo\repos\pwiz\pwiz_tools\Skyline\TestUtil\ScreenshotPreviewForm.cs:line 528
   at pwiz.SkylineTestUtil.ScreenshotPreviewForm.<>c__DisplayClass44_0.<FormStateChanged>b__0() in C:\Users\Eduardo\repos\pwiz\pwiz_tools\Skyline\TestUtil\ScreenshotPreviewForm.cs:line 265
   at pwiz.Skyline.Util.Extensions.ActionUtil.<>c__DisplayClass1_0.<RunAsync>b__0() in C:\Users\Eduardo\repos\pwiz\pwiz_tools\Skyline\Util\Extensions\ActionUtil.cs:line 61

[eduardo-proteinms]

@brendanx67 I also found that if the TakeScreenshot call in UpdateScreenshotsAsync throws an exception, the spinner keeps spinning. Maybe we need an indicator of error here.