Traktor library: fix importing track key

[remus32]

It looks like there used to be a KEY attribute in the INFO tag

mixxx/src/library/traktor/traktorfeature.cpp

Line 338 in 02150fa

key = attr.value("KEY").toString();

but Traktor 3.5.1 (the latest version) does not use this and has a MUSICAL_KEY node in ENTRY instead.

The keys are converted to string during import and are therefore immune to changes to preferred key notation in preferences.

[daschuer]

Thank you very much for this PR. It looks good to me.
Before merge, we need you permission to distribute your changes.
Please sign https://docs.google.com/a/mixxx.org/spreadsheet/viewform?formkey=dEpYN2NkVEFnWWQzbkFfM0ZYYUZ5X2c6MQ
and comment here when done.

[remus32]

Done

[daschuer]

Thank you :-)