message: fix(GUI): make the image selection window a modal
  The image selection dialog is attached to the parent window so it is a modal.
  Change-Type: patch
  Changelog-Entry: Make the image file picker attach to the main window.
diff: @@ -40,7 +40,8 @@ module.exports = function($q, SupportedFormatsModel) {
  */
  this.selectImage = () => {
  return $q((resolve, reject) => {
  - electron.remote.dialog.showOpenDialog({
  + const currentWindow = electron.remote.getCurrentWindow();
  + electron.remote.dialog.showOpenDialog(currentWindow, {
  // This variable is set when running in GNU/Linux from
  // inside an AppImage, and represents the working directory
  