Fix pointcloud properties dialog minimum size

[uclaros]

Description

The 3D View tab in a point cloud's properties dialog was forcing the dialog to relatively big minimum height.

Probably fixes #43276 , not sure about the disappearing part but I hope it was some QT weirdness when hitting the minimum size.

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 0b6ca1c)

[nyalldawson]

Nice approach!

Can I suggest for future proofness that we:

1. Add an enum:

enum class SizeMode {
    ConsiderAllPages, //!< The sizes of all pages are considered when calculating the stacked widget size
    CurrentPage, //!< Only the size of the current page is considered when calculating the stacked widget size
};

2. Add getters/setters for SizeMode, default SizeMode to ConsiderAllPages (ie match QStackedWidget)
3. Adapt the subclass sizeHint methods to consider the SizeMode

This way we could start moving everything over from QStackedWidget to QgsStackedWidget, and we'd have a natural place to add other tweaks and improvements in future. Much like what we do for QgsScrollArea. (There's also a test to force use of QgsScrollArea over QScrollArea, we could potentially add the same for QgsStackedWidget later)

[uclaros]

Hmm, I guess we could, I'm not sure about the potential gain (other than homogeneity). In most places we use a stacked widget it either holds similar widgets or it is used as a temporary container and the inactive page gets deleted.
With QgsScrollArea it was quite different, as it implemented the absolutely necessary functionality to avoid messing with widgets while scrolling.

[nyalldawson]

There's definitely value in this minimal height stacked widget elsewhere. There's lots of places where we have ugly empty space in dialogs because a stacked widget has forced a too large minimum height for the current page, and all the widgets under that get pushed down for no visible reason.

I'd still push for the enum setting added, because this is kind of our one change to do that and have the default behavior match QStackedWidget. If we need to add other tweaks to QgsStackedWidget in future then things get a bit messier as we'd need to avoid breaking the initial default behaviour.

[nyalldawson]

Let's expose, there's potential value here for plugins

[uclaros]

Should we maybe backport this as is and then expose and expand on a separate PR?

[nyalldawson]

I'm ok with that