feat: support fully custom preview

[mvarendorff]

Connection with issue(s)

Connected to #6 and #48

Solution description

This PR adds a new property to FormBuilderImagePicker that allows a custom layout for the preview. This creates an escape hatch for many problems like:

• Not being able to show images in their full size because the height is constrained by previewHeight
• Scroll overflowing like mentioned in Selecting too many pictures lets ImagePicker Icon disappear on the right #6 and Scrolling not working #48
• An even more custom layout like one of the solutions proposed under Selecting too many pictures lets ImagePicker Icon disappear on the right #6 (having the add button at the start and wrapping the images afterwards)

Screenshots or Videos

This for example is a custom preview that uses GridView.extent to lay the selected images out in a grid:

To Do

• Read contributing guide
• Check the original issue to confirm it is fully satisfied
• Add solution description to help guide reviewers
• Add unit test to verify new or fixed behaviour
• If apply, add documentation to code properties and package readme

[deandreamatias]

Nice contribution! Thanks for that

[mvarendorff]

@deandreamatias Is there a chance you could release a new version with this included? While I can just run the fork, they often get forgotten and for updates need to be rebased, etc. which is quite tedious and prone to error :/