Cleaned up pca

Separated functionality, allows for 1D-nav PCA (in a better way than
previously), and now prompts for conversion/copy (should also be put
into setting with "Do not ask again" option). PCA is now also threaded.
Remove mem leak

The standard MPL Qt backend leaks objects. Our backend implementation
was in large parts copied from that, so we were leaking as well, which
meant that Signals that had been plotted were never freed. This update
fixes that, although it was mostly solved by trial and error, so things
could pop up later.