This sets the unit that the attribute will be shown with. It
uses the same mechanism that's used to remember what units
you had selected.

It got mad if a layer was missing.

This way, people might actually read the instructions (:

Also fix the delete button appearing on page load

The ground now defaults to very-low, not bottom.
Zoom-on-add defaults to off.

This allows for multiple filters to be applied simultaneously. This commit
also moves some misc stuff, like the help and donate buttons, into a new
info menu. This also fixes touches outside of a popout menu not causing
the menu to close.