#483 Make document zooming more consistent in several ways

[Nathan Carter]

Although these could each be a separate ticket, I'm piling them together into one.

1. Lurch document content that's cut/copied to the clipboard goes on with a fixed font size. Zooming and then pasting pastes at the old font size. So instead store clipboard font sizes as a ratio of font size to document default font size, and convert back at pasting time, to keep the zooming illusion consistent.
2. Tab stops when zooming are already their own ticket, #480.
3. Just as zooming changes all font sizes in the document, it should also change all paragraph margins by the same ratio, to keep the zooming illusion consistent.
4. When zooming, also change the document's margins by the same ratio, or numbered lists end up falling off the left side of the visible area. In {{{wordprocessor.cpp}}}, you do {{{document()->setDocumentMargin( 20 );}}} and that number needs to scale with zooming.

[Nathan Carter]

Items 2 and 4 were done in [r3596][1]. Items 1 and 3 remain undone.

[1]: http://sourceforge.net/apps/trac/lurch/changeset/3596 (Solves ticket

480: Changing tab stop sizes when zooming.

Solves item 4 of ...)