Adding the ability to reorder metrics via the composer

This adds a new button menu on the right panel and new menu to the target context menu.
Gives the ability to select X targets, and move them in unison up and down.
If you select exactly 2 targets, then you can also do an in place swap.
Merge pull request #503 from justino/sortable_metrics_0.9.x

Adding the ability to reorder metrics via the composer