Port to GTK4+libadwaita

• ComboRow preferences don't work yet
• Basic buttons don't show when initially starting
• Financial mode dialogs need a refresh
• Buttons are sized differently (does calc() work differently?)
• Preference rows showing with underscore for shortcuts
• Currency conversion popovers are not scrollable and are too large
• SourceView doesn't work with dark style preference
• SourceViewCompletion needs to be re-implemented
• SourceViewCompletion ghost dialogs (see screenshot in comment)