- Use halign correctly ("left" and "right" aren't
valid values) and use hexpand in the
"Verify Encryption" button, since halign needs
hexpand set to "true"

- Ditch the spacer box, as it causes the page contents
to not scale correctly when the window width is small

- Add can-shrink property to "Generate New Password" and
"Verify Encryption" buttons so they can shrink when
the window width is small

- Also, set the margin-top only in the GtkBox to avoid
redundancy

Closes #2933