Restore ability to set font sizes in text=value format

In TinyMCE 3.x theme_advanced_font_sizes could be used to specify font sizes with text that differed from the value (e.g. '110=11px,120=12px' where 110 would appear in the font selection list and when applied 11px was the value used). fontsize_formats in 4.x doesn't support this.

This pull request adds support for specifying text=value font sizes in fontsize_formats.