#38 Add Instances: total width specification using mouse

[Ondřej Bouda]

The Add Instances function shows a dialog which allows the user to specify the total width, which the generated messages must fit in. The width is entered as a number, though. Specifying the total width using a mouse would be better.
After confirming the dialog with number of instances to generate, the user would drag a box (just like a selection) inside which the messages would be placed.