Relax XML schema for UIMA Index label

The current xml schema for UIMA specifies the value of the <label> part of the FSIndex descriptor for custom indexes must be the xml schema Name value https://www.w3.org/TR/2000/WD-xml-2e-20000814#NT-Name which doesn't allow, for example, blanks in the name.

One test case was actually using blanks (TextAnalysisEngineImplTest/MultipleAeTest4.xml), and so that part of the testing was being skipped.

As far as I know, the label value is only used as an identifier, so string should be fine.

Change the schema value spec for this to string.