[Radiant] fix the save button to the window

[eiked]

I'd like to suggest an change,
which I believe does ease editing in radiant CMS a lot.

I preffer to have the "Save" and "Save and Continue" (or Cancel)
Buttons
always at hands (without needing to scroll)

So I fix them to the lower right of the window:

public/stylesheets/admin/overrides.css:
   p.buttons { position:fixed; bottom:0px; right:4px;}