Use our own file open dialog

Clearly the static dialogs don't work. Even with the previous commit the
dialog still said "Save" insted of "Open". So let's just assemble our own
dialog and be done with it. I hope I got all the options right...

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Check error code of get_cylider_idx_by_use()

If not cylinder with type DILUENT or OXYGEN is defined, this function
returns -1 which should not be used as an index to an array. This
patch adds code to check for this return value and exit gracefully.

On line I marked with a comment. Someone more knowledgeable of that part of
code than me should double check that return is here what we want.

[Dirk Hohndel: fixed small oversight...]

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>