Use GtkWidget popup-menu signal in EtLogArea
Use popup-menu signal in EtLoadFilesDialog
Use GtkWidget popup-menu signal in EtBrowser
Avoid warnings when updating the scan dialog
Use same strings in GSettings schema and UI

While not all strings are applicable to the UI, try to reduce translator
effort by reducing the delta.
Move some GApplication handlers to EtApplication
Refactor about dialog handling

Fetch the parent from EtApplication.
Split off main() to a separate source file
Remove last uses of deprecated GtkStock
Remove use of deprecated GtkImageMenuItem