Replace overScaleMode with more flexible scaleMode

[joshkel]

As described at #612, I'd like to allow both regular zooming (to zoom both axes via mouse wheel or pinch on the main chart area) and zooming a specific scale (to zoom only one axis via mouse wheel or pinch over the scale axis), but it seems that overScaleMode prevents regular zooming from working.

I would have expected that mode control regular zooming, while overScaleMode controls scale zooming in particular. For example ("scenario A"):

• {mode: 'xy', overScaleMode: 'y'} would mean that zooming over the regular chart scales X and Y, and zooming over the Y axis scales Y.
• {mode: 'x', overScaleMode: 'y'} would mean that zooming over the regular chart scales X, and zooming over the Y axis scales Y.
• {mode: 'xy', overScaleMode: 'xy'} would mean that zooming over the regular chart scales X and Y, and zooming over the X or Y axis scales X or Y, respectively.

Instead, it appears that overScaleMode restricts and modifies mode ("scenario B"):

• {mode: 'xy', overScaleMode: 'y'} would mean that zooming over the regular chart scales only X, and zooming over the Y axis scales Y.
• {mode: 'x', overScaleMode: 'y'} is an invalid configuration.
• {mode: 'xy', overScaleMode: 'xy'} would mean that zooming over the regular chart does nothing, and zooming over the X or Y axis scales X or Y, respectively.

This PR adds a new property, scaleMode, that behaves like scenario A. Since scenario A is strictly more flexible than scenario B, it deprecates overScaleMode.

There is a potential change in behavior with this PR: Previously, the pan function used chart.scales if overScaleMode was undefined, and it evaluated directionEnabled (which called mode if mode was a function). This meant that it could handle changes to the mode function's return value, and changes to what chart scales exist, even in the middle of a pan operation. Now, the list of scales to check is only evaluated when the pan operation starts.