BtScan: rewrite as granite.messagedialog, gtk4 prep

[danirabbit]

• There doesn't seem to be a good reason to hold a ref to the application here?
• Use Granite.MessageDialog to save ourselves some layout pains
• Use modern functions of scrolled window instead of hardcoding a height
• Make sure dialog is resizable
• Code style
• Remove redundant list header

[ryonakano]

Also, the width of the frame is changed with this branch; now we have a wider left margin with the frame. Is this intentional?

Before

After