Don't rely on current path when searching for support files

The notion of current path changes as we open files in the file system.
What we really want is the directory from where Subsurface was started.
That covers both the case of Windows and running Subsurface from the
install directory.

This worked before because all support files were opened before the first
user interaction. But opening the manual showed the flaw in the previous
logic.

Fixes #348

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>