EditableText scroll with inset

[ickshonpe]

Objective

• Add an inset to the scrolling view so you get some look ahead when scrolling.
• The drag observer doesn't target the node's ContentBox.
• Pointer edits shouldn't automatically trigger scrolling, as it moves the view relative to the pointer.

Solution

• New component EditableTextNeedsScroll, newtypes bool. If set to true the editor view scrolls next update.
• Don't scroll on pointer edits.
• Scroll if dragging outside the text.
• New scrolling view calculations in bevy_ui::widget::text_input_layout::scroll_axis_with_inset.
• New scroll_inset field on EditableText controls the size of the view inset (is normalized).

Testing

cargo run --example multiline_text_inputs

Scrolling by dragging is too fast, and needs to be limited. Leaving it to a follow up, as this PR has a largish number of changes already.