#224 Fix bug when entering a relative path name (Unix)

[Anonymous]

When a relative path name is entered in the "Selection"
entry, it is not interpreted relative to the currently
selected directory, as I would expect, but relative to
[pwd].

Subsequently, the dialog gets confused and does not
allow browsing out of [pwd], because it now considers
[pwd] to be the file system "root".

This patch changes the behavior to be as I expect it,
by joining the text from the "Selection" entry with the
currently selected directory. This is consistent with
the behavior of tk_chooseDirectory on Windows.

Thanks to the semantics of [file join], the user is
still able to enter/paste an absolute path.

This patch also gets rid of the suspicious
set text [eval file join [file split [string trim
$text]]]
Although I do not understand the history of this code
fragment, it seems wrong, as it is incapable of
browsing paths that contain space characters: it fails
both when any of the path components contain a space,
and also if the user attempts to enter the name of a
directory that begins or ends in a space character.

[Nobody/Anonymous]

Fix handling of relative names