B&B editor: adding and removing steps

The 'Add/Remove steps' options on a pattern's context menu have been changed to always add and remove as many steps as there are in a full bar. The option to add or remove fewer steps is dropped.

Two new buttons are added to the B&B editor toolbar that perform these actions on all instrument tracks.

Rationale:
* The current way of adding new steps through a long context menu with a sub menu is awkward and requires precision. ([Fitts's law](http://en.wikipedia.org/wiki/Fitts's_law))
* Current options are 1, 2, 4, 8, and 16 steps. In projects that are not in 4/4 time, the number of steps in a bar is none of those. Adding steps involves navigating the menu multple times.
* There is also no benefit in adding fewer than a full bar's worth of steps, as the B&B pattern length is finally rounded up to the nearest full bar.