Spectrum: Allow frequency scrolling

[srcejon]

This patch allows a user to adjust center frequency up or down in a main spectrum by:

• Holding down the middle mouse button and moving the mouse left or right
• Moving the mouse outside of the spectrum area when dragging a channel marker.

For me, this makes it much quicker and easier to move to frequencies that are just out of range.

It works quite well on RTL SDR & USRP (for the latter, it's nearly as smooth as moving the channel marker) - but is a little slow on SDRplay.

Note, this works by GLSpectrum emitting a signal requesting a change of center frequency - The change of device frequency is handled by MainSpectrumGUI - so GLSpectrum doesn't need to know whether it is connected to a device or something else.

[srcejon]

On a different subject, did you mean to create the new waterfall.png and waterfall_3d.png icons so big? They're 480x480 pixels.

[f4exb]

On a different subject, did you mean to create the new waterfall.png and waterfall_3d.png icons so big? They're 480x480 pixels.

Looks like they were left at their original size when drawing them in Gimp.

[f4exb]

To be used with care... in a multi channel configuration the change in the device frequency affects all channels. You may want this or not.

[f4exb]

I think it is better if MainWindow takes care of device center frequency setting. Moreover it has access to the DeviceAPI object which can provide access to the virtual setCenterFrequency method of source and device sinks. This can provide support to a wider range of devices and maybe improve performance. I have corrected the code in that sense so that MainSpectrumGUI relays the signal of GLSpectrum adding the device set index for commodity and is later caught and processed in the MainWindow.

[srcejon]

Ok, sounds good. Thanks for updating the docs!