Remap `header-line` face to `topsy-header-line` in `topsy-mode`

[roshanshariff]

Many themes change the header-line face to be similar to the faces used by the mode line. This is often not a good fit for showing a sticky header. To allow the Topsy face to be customized independently of the usual header line, this PR remaps the header-line face to a new topsy-header-line face when Topsy is active.

By default, the topsy-header-line face just inherits from default, so it is displayed just as the rest of the buffer text would be. To revert to the original behaviour of using the header-line face, you need to do

(custom-set-faces '(topsy-header-line ((t :inherit nil))))

This PR is the first in a series that splits out the functionality of #7.