fixing planner's gas change dialog

Before this commit this dialog had a weird behavior in which
when it was shown, the whole screen was still active. In case of a
alt+tab, this dialog is sent to behind the application window, but still
visible in the task bar.

This commit basically changes this behavior to a regular pop-up.

Signed-off-by: Danilo Cesar Lemes de Paula <danilo.cesar@collabora.co.uk>