Support drag-and-drop in mcedit

[sergey-feo]

(excuse me for my bad english)

Drag-and-drop may sounds strange for old mc users. But it is expected feature for new users.

I suggest to implement it as it done in most modern text editors/processors (kedit, gedit, notepad, word, swriter).

Expected behaviour:

• press left mouse button on selected text;
• move mouse cursor -> some drag-and-drop indicator appears;
• unpress left mouse button -> text moved to new cursor position.