Implement toggle for egui::Window to only accept move events from the title bar.

[NotAPenguin0]

Closes #2774. For a more detailed description of the problem, refer to that issue.

demo gif.

List of changes

• Added egui::Window::movable_from_contents() to toggle the new behaviour on.
• In window::window_interaction(), split resize and move behaviour, since they might now use two different rects.

My apologies that there are so many commits, it's the unfortunate result of some dependencies I have to manage while developing. Just squashing on merge should get rid of the problem.

[emilk]

Suggested change

	/// Only allow moving by dragging from the title bar.
	pub fn movable_from_contents(mut self, movable: bool) -> Self {
	/// Allow moving the window by dragging its content?
	///
	/// Default: `true`.
	/// Regardless of this value, dragging the title-bar will move the window (if moveable).
	pub fn movable_from_contents(mut self, movable: bool) -> Self {

[emilk]

avoid negation in names

Suggested change

	self.move_title_bar_only = !movable;
	self.movable_from_contents = movable;

[emilk]

This seems like a lot of new code and complexity for a small change.

We should be able to just pipe in a smaller rect into move_and_resize_window?

[NotAPenguin0]

That is what I did initially, the problem is that this also moves the resize handle to a very weird and awkward position. I've also been able to implement this exact behaviour without touching egui internals at all, by simply doing

let remaining_size = ui.available_size();
let (response, painter) = ui.allocate_painter(remaining_size, Sense::drag());

and handling the drag value from this Response myself, so I don't think this change is needed anymore