Allow auto adjusting letterSpacing

[cxavier20]

Implements #73.

The letter spacing resizing is done when it reaches the MinFontSize and the text still does not fit. There is no need to resize the letter spacing earlier since resizing the text size already reduces the space between each character.

I did not add a maxLetterSpacing since I did not find it relevant. You can set the max letter spacing in the starting text style.