Enhance control panel for spectral plots with drawing settings and screenshot button

[ypnos]

Right now we have several means of manipulating the plot drawing but we only have some keyboard shortcuts to change them.

Here is a list:

• High quality / low quality -> drawHQ
• Framebuffer depth -> bufferFormat
• logarithmic alpha -> drawLog
• (drawMeans is not relevant)

See https://github.com/gerbilvis/gerbil/blob/496473f37ed1c57792c310eeb6746c83f33514c6/gui/dist_view/viewport_input.cpp#L185

1. These need also be accessible through the control widget. For the fb depth, maybe a slider is good, otherwise I suggest toggle buttons
2. We also need a screenshot button in the control widget (keyboard shortcut 's')
3. Write up a plain cheat sheet that lists all keyboard shortcuts

[ypnos]

I merged pull request #11 What is now missing:

• synchronize button/menu states with shortcut actions by using shared QActions