tz-launcher-wl: use DATADIR for the default icon path

Do not use a relative path anymore.

Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>
Fix all obvious bugs, adapt window size to icons, add a logic to
search for icons all around.

Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>