MainWindow: Fix build failing

[ryonakano]

Fixes #26

I'm on Horus Beta which comes with valac 0.56.3 and the build failing with the following message:

../src/Window.vala:48.3-48.19: error: Property `Wallpaperize.Window.file' with custom `get' accessor and/or `set' mutator cannot have `default' value
   48 |   public File? file {
      |   ^~~~~~~~~~~~~~~~~