#513 mouse and key binding for zoom and pan

[Mathias Zechmeister]

I would like to have more or other key bindings.
I set up a few here: https://github.com/mzechmeister/python/blob/master/zoom.gnu. It would be nice, to have those more or less built-in.
E.g.

• PageUp, PageDown -> as expected
• Ctrl+Home -> to go to first data point (GPVAL_DATA_X_MIN)
• Ctrl+Arrow -> horizontal/vertical zoom

Also nice would be :

• horizontal/vertical zoom with the mouse when on/below the axis
• horizontal/vertical pan when dragging with mouse on/below the axis
• mouse wheel -> zoom (currently vertical scroll)
• mouse wheel pressed+drag -> pan

Since user preferences can be different, those bindings should be more configurable.