Warnings as error

[alxvth]

Let's compile with /W3 warnings as errors on MSVC (with /WX). This will help prevent subtle errors, and we are pretty much warning-free on this level already anyways. I guarded this compile setting behind a CMake option MV_WARNINGS_AS_ERRORS which is ON by default.

This PR also fixes the last couple of warnings, which are all deprecation warnings:

• warning C4996: 'QtLiterals::operator ""_qs': Use _s from Qt::StringLiterals namespace instead
  • operator""_q is deprecated since 6.8
• warning C4996: 'mv::ProjectMetaAction::getSplashScreenAction': Replaced by application-wide splash screen customization
  • use Application::current()->getConfigurationAction().getBrandingConfigurationAction().getSplashScreenAction() instead
• warning C4996: 'ads::CDockWidget::CDockWidget': was declared deprecated
  • CDockWidget(const QString&, QWidget*) is deprecated, but internally just calls the other available constructor CDockWidget(CDockManager*, const QString&, QWidget*). We can do the same.
• warning C4996: 'mv::gui::WebWidget::[thunk]: __cdecl mv::gui::WebWidget::vcall'{360,{flat}}' }'': was declared deprecated
  • In Add web widget accessors #998 I deprecated WebWidget::initWebPage because we don't want to rely on it in the far future. But as is, many plugins currently trust this method to be automatically invoked. Let's revert this deprecation back to the previous notice, which will be easily greppable. We might want to document this need to update in all affected plugins similar to Update ExampleViewJS: do not use deprecated initWebPage() ExamplePlugins#45

Drive-by:

• In Project::fromVariantMap and Project::toVariantMap call _projectMetaAction.fromVariantMap(variantMap) and _projectMetaAction.insertIntoVariantMap(variantMap) instead of manually seralizing
• Do not skip the GroupAction serialization in ProjectMetaAction's serialization

[ThomasKroes]

From what I can observe, this change does not introduce any regressions. I tested the following scenarios:

• Opening existing projects
• Saving project A and reopening it in a new ManiVault session
• Creating a blank project and adding various (view) plugins
• Changing global settings