Filter NUnit Tree Display by Text

[rowo360]

This issue is another sub-item from #1145 (Redesign of GUI display) which propose to add filter functionality to the test tree.

This particular issue propose to extend the filter by some textual filtering.
The user can define some text filter in the UI (basically a string) and all tree node names matching the text filter will be displayed. The text filtering should support all parts of a tree node name: assembly, namespace, class and method name.

This functionality will be particularly useful for large test suites. For example if the user only wants to execute a single test and recalls some fraction of the test name, he can quickly select this test by this new textual filtering functionality.

[CharliePoole]

@rowo360 I gave it a Feature label and assigned it to the next milestone. You can do that when you create an issue, picking the appropriate label. Anything that's added to a milestone will usually be labeled as Feature, Enhancement or a Bug. This causes it to be picked up automatically and added to the eventual GitHub release.