[TxMt] Add ability to quickly change active theme

[Charles Stuart]

Hi,

Currently in order to change the active theme you have to go to the "View" menu, then the "Theme" menu item, and then select the theme.

It would be nice to be able to browse the themes quickly.

Either:

- keyboard shortcuts to change the active theme to the next/previous in the list.
- or, some sort of GUI that shows you all of the themes at once, sort of like the [previews on the wiki](http://wiki.macromates.com/Themes/UserSubmittedThemes)

Thanks,
Charles