Export Menu Option

[schneider42]

This adds a menu option to export (filtered) samples from a file.

If a tuner is activated (by adding a derived plot which uses it), the filtered samples will be used. The dialogue makes a recommendation for the decimation, based on the filter width.

Full file export uses multiple calls to SampleBuffer::getSamples() which leads do small discontinuities in the output file. I think this should be fixed in SampleBuffer::getSamples(). This is why I've added the experimental label to the option.

If cursors are activated, an option to only export the cursor selection is activated.

Things which should be improved (not necessarily in the PR):

• The FIR filter length is constant and sometimes not large enough for good results
• The complete file export is not "perfect"
• The recommended decimation might have an odd/unusual value
• Maybe there is a nicer and less invasive way to determine the decimation

Closes #49