Add one config.pro option to resize the datum plane, axis and coordinate system tags

[ptc-4658901]

For current functionality, unable to resize the datum plane, axis and coordinate system tags. The tags are very small  and even the color of coordinate system X,Y,Z axis is the same, which is very difficult to differentiate them.

[lxiao1]

It seems there is a hidden config.pro option text_height_factor for this.