"Copy [something] As Text" -> "Copy [something]"

Since it's not plain text, so could be misleading, and there's no need
to specify format as there aren't any other options.

Also corrected some first-time tips dialog text.
Readme updates.

Revised most of the usage content, apart from editing and settings
dialog text. Also switched primary font to RobotoDraft, and added some
more styled examples of UI elements. The changelog has also been updated
to reflect recent changes.