Implement the drag and drop framework

[daviunic]

Drag and drop needs to be implemented for the following scenarios:

• ListViewItems/TreeNodes are being dragged into the ListView/TreeView => move the items to the target directory
• ListViewItems/TreeNodes are being dragged out of the window into Explorer => extract the items to a temp directory and complete the drop so Explorer will move them to the actual target directory
• Files are being dragged into the window from Explorer => if no image is open, attempt to open the dragged file as an image. If an image is open, inject the dragged file(s) into the target directory inside the image

Most of this can be done even if the underlying image/FS functionality like inject, extract, move isn't in place yet.

[daviunic]

The overall framework is in place now. What's left that doesn't rely on underlying extraction code is to create arrays/whatever of selected ListViewItems/TreeNodes and temp path strings so that all selected files will be part of the operation.

[daviunic]

File extraction via drag and drop to Explorer has been implemented and merged into master. Same goes for opening images via drga and drop from Explorer when no image is open.

What's left to do is injection (from Explorer) and moving files/folder within the image (from ListView/TreeView to TreeView/ListView).

[daviunic]

Async extraction via drag-and-drop to Explorer has been implemented in the feature/async-extraction branch.

[daviunic]

Insertion and relocation within the image are waiting for the relevant FS code (write functionality).