ScrollArea improvements for user configurability

[MStarha]

• Closes ScrollArea adjustments for better configuration/customization #5406
• I have followed the instructions in the PR template

The changes follow what is described in the issue with a couple changes:

• Scroll bars are not hidden when dragging is disabled, for that ScrollArea::scroll_bar_visibility() has to be used, this is as not to limit the user configurability by imposing a specific function. The user might want to retain the scrollbars visibility to show the current position.
• The input for mouse wheel scrolling is unchanged. When I inspected the code initially I made a mistake in recognizing the source of scrolling. Current implementation is in fact using InputState::smooth_scroll_delta and not PassState::scroll_delta, therefore it is possible to prevent scrolling by setting the InputState::smooth_scroll_delta to zero before painting the ScrollArea.

A simple demo is available at https://github.com/MStarha/egui_scroll_area_test

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5443-scroll-area-adjustments
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.