drag/drop issues

[imario42]

On the Performance sample app I switched to week view and created a single entry.
When I am dragging and dropping this entry like a very nervous person I managed to leave back "ghosts" of the entry. Also when dragging/dropping the start or end time very nervous I saw some NPEs in the console.
This patch fixed all of these issues.

It came down to the fact that it is possible that you get mouseMoved events before the mouseReleased event even in drag mode.
In this case the initDragModeAndHandle set the dragMode to null too early and it looked like as if there was no dragging action active at all.
I also saw mouseDragged before mousePressed.

This patch introduces a boolean "dragging" which is set and unset in mousePressed/mouseReleased and, as long as this boolean is true, in mouseMoved the dragMode will not be reset.

I hope I was able to describe the issue good enough.

Java: Java 1.8.0_152-ea-b05
OS: macOS 10.13

[imario42]

Just in case: If it is not clear what is going on here, we can organize a TeamViewer (or something else) session and I'll show you.

[dlemmermann]

I was not able to reproduce this issue on my machine. Maybe we can Skype? My Skype handle is dlemmermann.