Add pickable attribute

[ffreyer]

This pr adds a pickable::Bool attribute that allows you to disable point picking for a plot.

My aim with this is to make pick(..., range) more useful. For example consider a line plot on an Axis. Currently there is a background mesh in Axis which will be picked whenever you're not exactly on the line. With this pr you can make the background not pickable, meaning that pick(..., range) has a chance to select the line as the next closest object.

In the context of JuliaPlots/AbstractPlotting.jl#677 this will also be helpful to avoid picking the information that's being displayed. (Probably necessary to avoid flickering if we display near the cursor.)

[ffreyer]

Works for:

• surface
• mesh
• meshscatter
• text
• lines
• linesegments
• heatmap
• image
• volume

[SimonDanisch]

Thanks, as discussed I would prefer to filter out the plots in AbstractPlotting, which shouldn't really come at any performance cost compared to doing this directly in the shader, with the benefit of making the shaders simpler and having this work across backends. The branch in the shader may even make this slower, than filtering in AbstractPlotting.

[ffreyer]

Ok, I'll close this in favor of #177 then.