Refactor extract conversation.PrintFileDialog

[gonzalo-bulnes]

Description

Moves widgets.PrintDialog to conversation.PrintFileDialog in order to make future changes easier to review.
Very similar to #1492

Removes the last reference to the non-GUI export module from gui/widgets.py. 🎊
Opens the way to uncoupling both export dialogs from the Controller, and encapsulating all references to that non-GUI export service into a single GUI object (interface if you wish).

Test Plan

• Confirm that the class hasn't changed significantly
• Confirm that the corresponding tests haven't changed significantly
• Confirm that the UI strings haven't changed
• Confirm that the print dialog looks the same as usual
• Confirm that you can still print files

Checklist

If these changes modify code paths involving cryptography, the opening of files in VMs or network (via the RPC service) traffic, Qubes testing in the staging environment is required. For fine tuning of the graphical user interface, testing in any environment in Qubes is required. Please check as applicable:

• I have tested these changes in the appropriate Qubes environment
• I do not have an appropriate Qubes OS workstation set up (the reviewer will need to test these changes)
• These changes should not need testing in Qubes because the UI would break even outside Qubes OS if anything was wrong

If these changes add or remove files other than client code, the AppArmor profile may need to be updated. Please check as applicable:

• I have updated the AppArmor profile
• No update to the AppArmor profile is required for these changes
• I don't know and would appreciate guidance

If these changes modify the database schema, you should include a database migration. Please check as applicable:

• I have written a migration and upgraded a test database based on main and confirmed that the migration applies cleanly
• I have written a migration but have not upgraded a test database based on main and would like the reviewer to do so
• I need help writing a database migration
• No database schema changes are needed

[gonzalo-bulnes]

Both commits have a green test suite 🍏, the failure is the Debian package build 📦.

I'll rebase the branch as soon as we fix the Debian package build.

[sssoleileraaa]

lgtm, clean refactor, much like the rest