Allow reordering graphs on worksheets via drag and drop

[Arkanosis]

Drag a graph, and drop it:

• between other graphs on the same sheet, to reorder the sheet's graphs without having to refresh the display several times (unlike today, when using the up/down arrows); [Edit by @fthevenet : done in v3.1.0]
• on another sheet, to move the graph from one sheet to another;
• on the “+” sign, to add a new sheet and move the graph to it.

Additionally, if #85 has been fixed, add the ability to move several graphs at the same time.