Avoid multiple calls to _extractTracesFromHTML in Trace2HTMLImporter.

For each HTML import, we trigger multiple calls to _extractTracesFromHTML.
In Trace2HTMLImporter.canImport() and in Importer.extractSubtraces().
We can instead cache the value of subtraces also during canImport and
return the cached value during extractSubtraces().
Merge pull request #620 from kphanee/trace2html_avoid_multiple_calls

Avoid multiple calls to _extractTracesFromHTML in Trace2HTMLImporter.