Improve code quality of picker.mjs

[plhrtr]

• Add a comment with more info on the normalization in mouseposition()
  • presumably: normalization to [-1, 1] and inverting the y axis to go from DOM to three.js world space
• Simplify if-statements in updateSelectionBox(), attachSingleTransformControl(), and updateSelection()
  • reduce unnecessary nesting of if-statements and merge if-branches with the same outcome/shared code
  • set showX = (axis === "X") etc directly if possible
• Consistence use of this.#selectedObject, this.#transformControls.object, and this.#selectedObjects[0]
  • Should they be identical (in most cases) or are there significant differences?
  • If identical → is it sufficient to just use this.#selectedObject?
  • If not identical → clarify how they differ
• Check if deselectAll() should reset the this.#selectedObject
• Should itemSelectedEvent() be renamed to dispatchItemSelectedEvent()?
• In setUpMouseEvents(): find a better name for canvasChild or add a comment explaining what exactly the last child of canvas is
• Can we use this.#canvas in itemSelectedEvent()?