Improve data path handling

Originally I intended to switch to QStandardPaths for Qt5, however this
would also require migration code. We'll postpone this until we can
get rid of Qt4 and KDE4, so there are less combinations to handle properly.

Instead, this commit improves the handling of data paths in general by
fixing search order, supporting XDG_DATA_HOME, and sanitizing the paths
properly. Also KDE builds now check their own install prefix, in case
Quassel is installed in a non-standard location.