Use Granite.MessageDialog instead of Gtk.MessageDialog

[ryonakano]

Before

After

Changes Summary

• Use Granite.MessageDialog instead of Gtk.MessageDialog
• Add "Cancel" button in case users clicked the "New Timetable" button by mistake (Indeed we can cancel with Esc key, but I think we should clarify)
• Make buttons in the filechooser translatable

I'm worried that the "Save" button in the MessageDialog is not focused when the dialog is shown, because if users press Enter key wrongly without switching the focus, they should lose their timetable. But I'm not sure how to bring the focus to the "Save" button, would it be possible for you to tell me how?

[lainsce]

I think you can override focus, but I'd need to research in Valadocs. I'll edit this comment with the process later. As for the primary text, can you remove the "Do You Want" part so it says "Save changes to the Timetable?" ?

[ryonakano]

Addressed

[ryonakano]

@lainsce Searching in Valadoc more deeply, I've found how to set focus to the save button by default !! I addressed it in ce878ad2b06ffab0b639a915fcdd809202ed042c

[lainsce]

All lookin' good! Merged!