feat: Enabled loading of FXML files via drag&drop to WelcomeScreen.

[Oliver-Loeffler]

This feature PR introduces drag&drop to the welcome page.
One now can select a group of files and directories and drag them over the welcome page. On drop, the files are filtered to be FXML files and dropped subdirectories are searched for FXML files in first level.

All FXML files found will be passed to the file open process of Scene Builder.
For unsupported files or empty directories no message will be displayed as long as at least one file can be loaded.

If no file can be loaded at all, a message is displayed to the user that there were either unsupported files and/or the selected directories did not contain any supported file.

Issue

Fixes #584

Progress

• Change must not contain extraneous whitespace
• License header year is updated, if required
• The PR name must follow the pre-defined format
• Verify the contributor has signed Gluon Individual Contributor License Agreement (CLA)

[abhinayagarwal]

Can we also make sure that Scene Builder doesn't exit if an invalid FXML is dropped or opened?

[Oliver-Loeffler]

This is partially handled by PR #576, #577. I will look into what else is needed and issue a separate PR if it's complex enough. If its a small change, it might become part of this PR.

[Oliver-Loeffler]

Hi @abhinayagarwal,

Thanks for reviewing. I was wrong with my previous statement and after some reading #585 turned out to be the relevant PR for this. Please have a look at PR #585, this one exactly addresses the issue of error reporting and keeping the Welcome Page open. Its a bit more complex but this is IMHO a good basis for #591. Both combined work quite well.

My proposal is to merge PR #585 first, then get the drag&drop one merged.
Looking forward to your feedback regarding #585.