Update relation logic to work for composite type as well

[xkello]

Description

Fixes #2315

The app treated every QGIS relation as an association, so deleting a parent never deleted its children. Children added while creating a new parent were also left behind as orphans when that parent was discarded. Relation strength is now read from the QGIS project, and composition relations are respected.

What changed

• InputUtils::deleteLinkedFeatures() deletes all features linked to a feature, including nested ones. Normally it only follows composition relations, but when a new feature is discarded it follows every relation.
• InputUtils::hasCompositionChildren() tells whether deleting features would also delete linked features.
• AttributeController::deleteFeature() and MultiEditManager::deleteSelectedFeatures() delete the linked features first and then continue with their original delete flow.
• AttributeController::hasCompositionChildren() and MultiEditManager::selectedFeaturesHaveCompositionChildren() drive the confirmation text.
• MMFormDeleteFeatureDialog.qml gets a deletesLinkedFeatures property that switches to the new description.
• MMFormPage.qml deletes everything linked to a new feature when it is discarded or deleted in add mode.

Behaviour

• Composition: deleting a parent also deletes its children, grandchildren and further levels.
• Association: unchanged, deleting a parent keeps its children.
• New parent discarded without saving: children created while adding it are deleted, for both strengths.
• Before deleting a parent with composition children, the dialog says the linked features will also be deleted. This works for one feature and for several selected features.

[github-actions]

Coverage Report for CI Build 37489113905

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📦 Build Artifacts Ready

OS	Status	Build	Info	Workflow run
macOS Build	📬	Mergin Maps 73871 dmg	Expires: 04/01/2027	#7387
linux Build	📬	Mergin Maps 74131 x86_64	Expires: 04/01/2027	#7413
win64 Build	📬	Mergin Maps 65891 win64	Expires: 04/01/2027	#6589
Android Build	📬	Mergin Maps 869911 APK [armeabi-v7a]	Expires: 04/01/2027	#8699
	📬		Google Play Store	#8699
Android Build	📬	Mergin Maps 869951 APK [arm64-v8a]	Expires: 04/01/2027	#8699
	📬		Google Play Store	#8699
iOS Build	📬		Build number: 26.10.963911	#9639