Add separate settings dialog to configure 'Test Run settings'

[rowo360]

This PR fixes #1406.

Bases on the discussion in the issue, I split this task into two commits:

1. Removal of these TestCentric application settings: ShadowCopy, MaxAgents and PrincipalPolicy
   That means there are no user specific default values anymore, when creating new projects. But instead the NUnit default values are always applied to new projects. Only the setting ShadowCopy is always enabled for new projects (NUnit default is disabled). As a consequence, the settings dialog is retrieving the values from the test project now, instead from the application settings.
2. In order to make it clearer to the user when which settings are saved, I split the settings dialog. The 'Assembly load setting' setting page, which configures those three settings, is removed from the general settings dialog. Instead it's displayed via a new menu item 'Test Run Settings..'

Here are two screenshots:

I can well imagine that we will also transfer the configuration of setting RunAs86 and SelectedAgent to this new dialog. Then these related settings would all be together. But that can be done at a later point in time.

[rowo360]

Ups, because of our exchange, I completely forgot about this PR - so I merge it, now.

[CharliePoole]

I didn't know there was a PR and while I'm glad you did it, I would have liked to review it but you would have to request the review the way things are set up. That's the process I've always used in my own teams, but I know some people prefer to avoid this by having it done automatically. So, if you like, I can have GitHub make the assignments automatically. Since we are a team of two people, it would always "select" me to review your PRs and you to review mine. We both have the authority to override the review and merge anyway if necessary.

In any case, I would only have had some minor suggestions on this one, so thanks for getting this done.

The X86 and selected Agent settings are a bit different because they can apply to individual assemblies. I didn't want to complicate things by adding UI for that situation until the other things we have discussed are done. But I'll put something into an issue about it so we have can track it.

[rowo360]

Sorry, I did the merge too quickly. If you have any suggestions (even minor ones), I will change the code accordingly even now. And next time, I will start the review process right from the start.

[CharliePoole]

I'll wait until after we do the PR for #1413. If you want to do that one, you can assign it to yourself. There is some degree of risk of conflict with #1414 but I don't think it's too high as the major changes will in the model rather than the UI. There will be some for the one who merges last of course. :-)