fix: enable Scene Builder to load FXML with unresolved imports (which are still preserved)

[Oliver-Loeffler]

This fix allows Scene Builder to load FXML files with unresolved imports. There is also a new option (keep unresolved imports) which also allows to keep the imports in saved FXML files. This can be toggled in Scene Builder preferences dialog.

If disabled, Scene Builder will effectively omit those unresolved imports and remove them from the updated FXML to be written. Of enabled, Scene Builder will keep track of unresolved imports and write them (unmodified) into the updated FXML. The import remains as it was declared in the original FXML file. Optimization is not applied to unresolved imports.

Issue

Fixes #120 Fixes #281

Progress

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