KeyFilter: extend the appearance algorithm to resolve display properties against _all_ matching filters, not just the first. The most specific filter to set a given property will win. This results in a much more intuitive display. Among other things, it allows one filter to set the font the bold and the next to set it to strike-through, and their requests are merged.

NB: I intended to use QFont::resolve for this, but that didn't work and expected surprising results, so I had to code a FontDescription class that does what I want instead.

svn path=/branches/kdepim/enterprise4/kdepim/; revision=986555