Scaling for plots, band display in plots and calibration assistant

This release of NanoVNA-Saver offers a number of new features, all of which have been widely requested.

First of all, there's now the option of scaling the plots: right click them for a menu, where it's possible to set the maximum and minimum values for the frequency and data axes. Sadly not yet ready for the polar plots, and mouse control of zoom is also pushed to a future release.

Second, it's now possible to have "bands" displayed in the frequency based plots: Select "Display setup" to find the option for this. The default data is for amateur radio bands - or you can put in your own.

Thirdly, the calibration procedure has received a new "calibration assistant": A series of popup messages prompting you to switch between calibration standards, and code to automatically sweep them for you and store the results.

Finally, there's as always a number of bugfixes and stability improvements. I look forward to hearing what you all think about this 0.0.12 release!