Change how margins are set

Now the user can only control the left and right margins, since all he
wants to do is set the text width to be comfortable.  But we ensure a
minimum of 1em around the content so there's a little space for it to
breathe.  Next, we'll set the top and bottom margins, and the default
side margins, based on the screen size.
Set default margins based on screen size

Right now, we aim for a text width of 60gu, about 35em or 70 characters.
Add toolbar icons

Most are from the Ubuntu-Mobile icon set.  I reference them by path,
which is probably not optimal.
Strip extra spaces out of ToC entries

Some books, especially in Epub3 format, assume spaces collapse in ToC
entries.
Use gradients for dialog buttons

Since that seems to be the default.
Attempt to sort by authors' last names

This can't be perfect, since epubs just give us a "creator" string with
no info on how to interpret it.  We assume one comma means "Last,
First", otherwise it's "First Last".  Multiple commas mean it's
something else, and we don't try to guess what.