Replace fontSize buttons with increase/decrease/textbox

[vekt0r-github]

Draft of a feature that replaces the set of predetermined font sizes with the following controls:

• A display of the current font size (doubling as a entry point to custom input)
• A pair of buttons to decrease and increase the font size, preferring to snap to certain values that increase exponentially (16*1.2^n)
• When the first button/display is clicked, the previous buttons are replaced by a text input to directly set the font size (disappears on blur)

Current draft of interface:

If desired, a similar interface for line widths may address https://github.com/edemaine/cocreate/issues/172.