Make sure that text lines aren't squeezed together in new Qt versions

Make sure that text lines aren't squeezed together in new Qt versions

In recent Qt 4.6 versions, there is a commit[1] that changes behavior wrt to lineSpacing() and
friends. Since (at least on my box) the font leading (the distance between text lines) seems to be
always -1 (for reasons as of yet unbeknownst to me), using lineSpacing() lets text lines in ChatView
overlap. This workarounds this with using at least height().

[1] http://qt.gitorious.org/qt/qt/commit/04d18b38c38c5ff623b30366ea08d56128b9b7d0