ctest: support <CTestMeasurement> for runtime measurements

Teach CTest to parse output for <CTestMeasurement> in addition to
<DartMeasurement> for measurements defined at runtime.

Use a new class (cmCTestTestMeasurementXMLParser) derived from cmXMLParser
to parse the data and attributes these XML elements. This is an improvement
over our previous approach of using a series of regular expressions.

As part of this commit we also rename some member variables and methods
to make their purpose more clear.

DartStuff                        -> AllTestMeasurementsRegex
DartStuff1                       -> SingleTestMeasurementRegex
DartString                       -> TestMeasurementsOutput
GenerateDartOutput()             -> GenerateCTestXML()
GenerateRegressionImages()       -> RecordCustomTestMeasurements()
cmCTestRunTest::DartProcessing() -> ParseOutputForMeasurements()