Update our published QML code to reflect current best practices

This applies to Qt source code, example source code, and documentation snippets.

Examples:

• Replace "property var" or "property variant" with the strongly-typed equivalent where possible
  • property var items -> property list<string> items: https://github.com/qt/qtdeclarative/blob/v6.8.1/src/quickcontrols/doc/snippets/qtquickcontrols-spinbox-textual.qml#L14
  • property var easingCurve -> property list<real> easingCurve: https://github.com/qt/qtdeclarative/blob/v6.8.1/examples/quick/animation/easing/easing.qml#L14

• Replace injected "model"/"modelData" properties with required properties to avoid unqualified access. UPD: moved to https://bugreports.qt.io/browse/QTBUG-133272 
  • https://github.com/qt/qtvirtualkeyboard/blob/v6.8.1/src/styles/builtin/default/style.qml#L786

• Apply QTP0001 instead of manually setting RESOURCE_PREFIX "/qt/qml"
  • https://github.com/qt/qtquick3d/blob/v6.8.1/examples/quick3d/customgeometry/CMakeLists.txt#L36

• Apply type annotations to JavaScript functions, for AOT compilation
  • function move(mouseX) -> function move(mouseX: int): https://github.com/qt/qtvirtualkeyboard/blob/v6.8.1/src/components/AlternativeKeys.qml#L96
  • function append(text) -> function append(text: string): https://github.com/qt/qtdeclarative/blob/v6.8.1/examples/quick/animation/behaviors/wigglytext.qml#L25

• JavaScript: Use strict (in)equality (=== or Unable to render embedded object: File (==}} instead of {{==}} or {{{}) not found.=) where types are well-defined
  • https://github.com/qt/qtdeclarative/blob/v6.8.1/examples/quick/customitems/painteditem/textballoons.qml#L28-L30
  • https://github.com/qt/qtvirtualkeyboard/blob/v6.8.1/src/components/Keyboard.qml#L1776