Remove Awkward Stretching of Config Widgets

When the config tool's window size is stretched, the components stretch
awakardly (ex. 4 lines will be evenly distributed over the entire
height of the window). This commit fixes this by adding a stretch
factor of 0 to the main layout.

Close #518
Related to #411
Signed-off-by: Thanh Ha <thanh.ha@alumni.carleton.ca>