Create B_DEFAULT_FONT_SIZE or some other such constant

[waddlesplash]

A good amount of the scaling code is independent of font size entirely, and computes margins purely based on what it is, e.g. (font size / 2) for 6px margins. However, some code requires an actual "scaling factor", and this code presently hardcodes 12.0f for this purpose. Instead a constant should be created, e.g. B_DEFAULT_FONT_SIZE, as suggested by X512 in #13170.

[waddlesplash]

I don't think we need to do this after all. More and more places in the code are being changed to use BControlLook spacing calculations instead of "font size / 12", and if we really did need it, we should instead add the global scale factor elsewhere.