New layout command: Make into system

Currently (v1.0), the only way to control system breaks is a combination of:

-manual system breaks: to force breaks where you want them
-"Add more stretch"/"Add less stretch": for getting rid of automatic breaks in place you don't want them to be

Manual breaks are nice, but the "stretch" commands are not very intuitive. In Sibelius there is a command that combines these two: Make into system. This command forces the selected measures to stay on the same system (if possible) and forces a system break before and after the selection. The command does several things at once, and it's easy to figure out what it does just by reading its name in the menu. In MuseScore 1.0, the equivalent would be: manual line break, increase stretch (several times may be required), manual line break again. And it's not that easy to tell what "Add more stretch"/"Add less stretch" does, when looking through the available commands in the menus.

No matter what, I feel that "Add more stretch"/"Add less stretch" needs to be renamed. First of all "Add less..." makes no sense, so "Increase stretch"/"Decrease stretch" or "Stretch more"/"Stretch less" would be better. But ideally, they would have a name that reveals more precisely what they do, eg. "Stretch measures"/"Shrink measures".