Implement Drag and Drop for X11

[ssiegel]

This implements accepting files/folders via drag and drop as on the
other platforms. It adds receiving URLs dragged e.g. from a browser and
plain text.

For filenames and urls an additional space is inserted after the last
item to enable adding more files and urls with another drag-and-drop
operation without the need to manually enter the space in between.

This is the last platform to resolve #640

[ssiegel]

The change to add the space after the dropped files/urls might be up for debate, but it is what I'm used to from Konsole. I don't see how an additional space would be undesirable anywhere where I might want to drop filenames. Even if there might be a situation, then deleting it with a single backspace seems easier to me than having to change focus and press space between multiple drag-and-drop operations.

[wez]

Thank you!