imenu does not index interface-prefixed method declarations.

[josteink]

Example code:

class ImenuMatcherTests : IDisposable
{

...
    void IDisposable.Dispose()
    {
        Console.WriteLine();
    }
}

Dispose() does not get indexed.

Currently this works for interface-prefixed properties, so the support is somewhat inconsistent...

Fix this, consider performance when writing regexps and create a better test-suite covering a wider array of imenu tes-cases.