Drag and drop PNG artwork into the app icon editor

Allow PNG files to be dragged from the system file manager into Studio's application icon editor.

Drop a PNG onto the default artwork preview to set the default artwork, or onto an individual size slot to supply artwork for that size. Show a clear highlight for valid drop targets and a useful error when a file is invalid or does not match the selected slot's required pixel dimensions.

This makes it quicker to add an existing set of specially prepared icon sizes without opening a file picker for each one. Keep the existing file-picker buttons available.