Position the paste target just below the top of the screen

The paste target has a fixed position, and was at the top
of the screen (top: 0). This change moves it down a little
bit (1em) so it doesn't cause scrolling when it's typed in.

This fixes a few bugs that I couldn't find in Bugzilla:
* Typing while a focusablenode is selected scrolls the page up a little
* Copying and pasting scrolls the page back and forth

Change-Id: I85ad9e4e0e85b99825bf5322e621cbd74d80f21d
Merge "Position the paste target just below the top of the screen"