Make all attributes of TextStyle optional

[Aran-Fey]

Some attributes of TextStyle default to None, which means "unchanged" (i.e. inherit the value from the theme or whatever), but all attributes should do this. This is a tricky change because it affects quite a few different parts of rio, including:

• Theme
• getTextDimensions (extra tricky because it removes the element from its parent)

[Aran-Fey]

This is actually currently bugged because the font is already optional, and getTextDimensions doesn't handle that case correctly.

[mad-moo]

Duplicate of #59?