app-window: Make stacks, homogeneous

This is so that Boxes window doesn't suddenly grow back on its own
after user has put it in a particular size.

Turns out that I was wrong in commit
fc218e0904c6cac39de46e2f4138a3739b7e54d7, where I assumed the opposite.

https://bugzilla.gnome.org/show_bug.cgi?id=720908
wizard: Reduce vertical margins

This is so that main window can fit in 625x750px. However we still won't
fit on many small screens out there, where height is 768px and
gnome-shell's topbar already takes 24px.

https://bugzilla.gnome.org/show_bug.cgi?id=720908
wizard-source: Show 4 medias on short screens

On small screens, where height is typically less than 800, lets keep
number of visible medias limited to 4 (rather than 5). This is so that
main window can fit in 625x678px.

https://bugzilla.gnome.org/show_bug.cgi?id=720908
wizard: Put some margin on the left & right

So wizard doesn't look horrible on small dimensions.

https://bugzilla.gnome.org/show_bug.cgi?id=720908