Tab size should be computed in space units, not em units

ST2 currently computes tab size using em units (or something else that ends up measuring 1 em) instead of spaces. This matters with proportional fonts, where a space is this wide: | | while an em unit is this wide: | |.

Setting Tab width to 4, for example, should indent the text by a width of 4 text spaces which is this: |  |, instead ST2 indents by 4 em units, which is way too wide: |    |.

See this forum post.