Add support for antialias and related subpixel order settings

[jhaygood86]

This adds selectors for antialiasing, and if subpixel antialias is chosen, the subpixel order. It actually renders the labels within the mode buttons using the text settings selected so the user can see how it will look in advance and select the best looking version.

Example screenshot:

Fixes #288

[kdwk]

I think the spacing between the newly added options should be increased, because now the options seem too closely packed

[jhaygood86]

Added some margins between the new settings.

[cassidyjames]

My inclination with these additional text-related options is that we should move this layout over to a sidebar with:

• Wallpaper
• Appearance (or Style?)
• Text (is that the best name?)
• Dock & Panel
• Multitasking

Does that make sense @elementary/ux?

[danirabbit]

@cassidyjames yeah ultimately moving to a sidebar would be better imo. For the purposes of this PR, it would probably be good just to split the text options off into another tab

I'd also be inclined to use radios for these options instead of modebuttons

[jhaygood86]

Added a new "Text" tab and moved text related settings there. Switched mode buttons to radio buttons for new settings