fix: SceneBuilder will not load missing files from recent projects.

[Oliver-Loeffler]

Scene Builder will ask to remove missing files from recent files list. Welcome Page will not close, there will not be an error message.

Issue

Fixes #582

Progress

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[Oliver-Loeffler]

https://user-images.githubusercontent.com/22102800/194958675-a96fac1a-88f9-4c0f-b90b-588389bae6a3.mov

Window positioning is not yet okay. I am hesitating to touch the AlertDialog for this. Something like positionCentralOver or so could be helpful.

[abhinayagarwal]

The new tests added to the app project doesn't seem to run.

[Oliver-Loeffler]

I've moved the surefire-plugin from kit/POM.xml to POM.xml so that now tests of app and kit are executed during Maven build.

[Oliver-Loeffler]

This PR now additionally addresses issue #600 which describes the behavior of unintended closing of SceneBuilder.