Sanitise filenames in UploadAction

[medienbaecker]

Fixes filenames with special characters (like +) that break the Panel by using Kirby's F::safeName() method, consistent with how Kirby handles uploads in the Panel itself.

Closes #272.

[mzur]

Thanks! Could you please also add a test case for this? Here is an example.

[medienbaecker]

I've added a test for the new feature, inspired by your existing tests (first time writing tests for me!).

I've also declared the $dir and $form properties to fix PHP 8.2+ deprecation warnings about dynamic properties. These properties are used in setUp() and in the existing tests. Without the declarations I got "Creation of dynamic property is deprecated" errors.

Let me know if this is all correct and thanks for your work on this great plugin.

[mzur]

Thanks!