spectral_analyzer: Improve sonogram display

[JoepVanlier]

Made a few additions to the sonogram display.

• There is now a right click context menu that allows you to choose various display options
• There is an optional frequency grid.
• Right click context menu can be used to switch between log and non-log display.
• Scrollwheel still enables zooming into the low frequencies.
• Left click still drags the scale, for which there is now visual feedback.
• The color mapping was also overhauled and is now done logarithmically, to be more in line with the rest of the analyzer.