FolderView: Performance improvements for selections

This MR has multiple changes, especially related to the "rubberband" (selection rectangle).

• Use Qt.callLater for gridView.rectangleSelect to eliminate redundant calls.
  • This was previously called every time mouse moved one pixel
• Clear a redundant mapToItem
  • var cPos = mapToItem(gridView.contentItem, mouse.x, mouse.y); needs to be only called during rubberband action
• Update dragItem images after selection is done, instead of every time a delegate is selected
• Instead of creating and destroying selectionButton in FolderItemDelegate, create it once and toggle its visibility

These all should help with the performance related to selecting multiple icons by dragging the selectiong rectangle around them.

BUG: 493376