Implement selected text deselect by mouse and selected text drag&drop in same time

It will be better to implement selected text drag and drop like in other editors, for example visual studio.

1. Select text
2. Mouse down on selected text
3. If mouse moves out of 7x7 pixels box from the initial click position drag&drop is started
4. If mouse up is happened while mouse still inside the box text just deselects

This variant is more user friendly and cover both user operations.