Tweak resize behavior to avoid text cutoff

[eloquence]

Status

Ready for review

Description of Changes

Fixes #597. Enforces a maximum and minimum size while allowing text to take up as much space as needed, even if font size changes.

• Assign a grid layout to the QDialog to ensure it reflows with resize
• Match QDialog class to generated UI code in multiple inheritance
• Set size policy for long text to "minimum" to ensure it always allocates as much space as needed
• Set a maximum to allow for some adjustment by the user

Testing

Default font size:

1. Apply the changes in this PR to the launcher versions in /opt/securedrop/launcher and /srv/salt/launcher (if only the /opt copy is overwritten, the updater itself will replace it on the next run).
2. Run /opt/securedrop/launcher/sdw-launcher.py --skip-delta 0. This forces an updater run.
3. While the updater runs, attempt to resize the window. Note that you can do so up to a maximum and minimum size.

• Observe that no text has been cut off.

Increased font size:

1. Under System->Appearance in dom0, increase the "Default Font" to a very high value, e.g., 17.
2. Re-run the updater as above.

• Observe that no text has been cut off.

Checklist

• Linter (make flake8) passes in the development environment

[sssoleileraaa]

• Observe that no text has been cut off.
  Increased font size:
• Observe that no text has been cut off.

Nice fix