Scale tool: add support for non-uniform scaling, controlled by Shift key

[elektronisk]

Here's a commit that fixes #195. It lets you scale with different amounts horizontally and vertically if you hold the shift key while scaling.

It generalizes the existing logic to be able to operate on x and y directions independently.

I'm unsure what to do with the absmax function, if it should be a static ScaleTool method, part of some utility function collection, or if it's ok for it to just hang out where it does as it does now. Please advise on this.

[elektronisk]

Screen.Recording.2025-06-04.195843.mp4

Here is a short clip showing the function.

[dl3sdo]

Maybe you want to add the possibility to cancel the scaling operation with Escape key (as added by @dg0yt in #2360).
Aborting some other operations (e.g., rotating) with Escape is not (yet) possible whereas aborting a move operation is.

bool ScaleTool::keyPressEvent(QKeyEvent* event)
{
	switch (event->key())
	{
	case Qt::Key_Control:
		using_scaling_center = false;
		updateStatusText();
		updateDirtyRect();
		return false; // not consuming Ctrl
	case Qt::Key_Shift:
		uniform_scaling = false;
		updateStatusText();
		updateDirtyRect();
		return false;
	case Qt::Key_Escape:
		if (isDragging())
		{
			cancelDragging();
			if (editingInProgress())
				abortEditing();
			updateStatusText();
			updateDirtyRect();
		}
		return false;
	}

	return false;
}

BTW: I propose to move double absmax(double v, double max) into an anonymous namespace.