Design improvements

[onerbs]

• Fixed: There was a slight change in header height when the Back button appeared.
• Changed: The Radio Buttons are shown in column instead of row in Split page.
• Changed: Show a wider Merge button in Merge page.
• Added grid.set_column_homogeneous(true); for Compress and Convert pages.
• Granite.Application is deprecated. Using Gtk.Application instead.
• Another small design changes...