[Refactoring] Check context action mimetype against file

Fixes error spew from the F# refactoring tools when editing C# files.
[NUnit] Use more robust filename escaping

Instead of simply stripping out path chars, replace them with
percent-encoded values. This means we no longer incorrectly
conbine configurations's stores e.g. Debug|Foo and DebugFoo.

Based on MSBuild escaping in the MSBuildService.
[Nunit] Use escaped configuration names when finding stores

Prevents "invalid path" errors on Windows that messed
up the Units Tests pad really badly.