Show/hide test duration in test nodes

[rowo360]

This PR close issue #1137 by displaying the test duration in the tree nodes.
The test duration is shown in square brackets behind the tree node name.

The user can turn on/off this feature using the tree context menu:

The current state (turned on/off) is saved in the settings, so that the last active state is restored when restarting TestCentric. The state is not stored in the VisualState file.

From technical point of view the TreeNodes are created by the DisplayStrategy class, therefore I placed the code there (the core of the implementation is done here). It handles these use cases:

• OnTestFinished: update tree node name of the finished test
• OnTestRunStarting: reset all tree node names - that's identical to our behavior regarding the tree node images. So that no outdated results from a previous run are shown.
• Turn on/off features: update all tree node names
• Switch display strategy/grouping: If feature is turned on, the new strategy/grouping will display test durations right from the start.

If no test results are available or the feature is turned off, the tree node names remain as before.

[rowo360]

@CharliePoole :
I'm aware that we are already focused on the next issues. But may I draw your attention to this PR?
It would be great if we could finalize this before I start with the next issues and potential merge conflicts.
And some review is required as the next step - thanks a lot!

[CharliePoole]

Right! I held off till after the release and then forgot it.