Move pqTransferFunctionChartViewWidget::plots<T>() to header

This moves the definition of the plots<T>() template method to the
header file. This was causing a compilation error on Intel.

Thanks to Nico Kruithof for finding the error.

Change-Id: I1f2f98e44855dcadeef34f5dd89566d8c036296b
Merge topic 'transfer-function-plots-bugfix' into next

d39ca5f Move pqTransferFunctionChartViewWidget::plots<T>() to header