Soft hyphen cleanup

Redo the hyphen insertion a little bit.

We no longer mark soft-hyphen breaks in log attrs, instead we look at the surrounding text to see if we need to insert a hyphen. Currently, we just look for the soft hyphen, but with some more work, this could be expanded to do full TeX-style \discretionary{k-}{k}{ck} breaks.

If we do that, we need to add the pre- and post-break text the existing items and re-shape with it though. And to do that conveniently, we need some way to pass text in chunks down pango_shape(). That would get a little complicated:

Say, we have:

paragraph_text: paragraph before blackbla paragraph after
item text:                before blackbla

We would need to pass the following chunks to pango_shape():

paragraph_text: [paragraph before bla][k-][k][bla paragraph after]
item text:                [before bla][k-]

Or do we need to keep ck in the paragraph text?

Another question is how to mark such breaks - with an attribute ?

bla<span prebreak="k-" postbreak="k">ck</span>bla