fix doc about LiveComposite
fix doc about MoveTransition
Add the uses_scope property to Text.

This tells us if we need to update the text when the scope changes.
Merge pull request #379 from kyouryuukunn/fix_doc

fix doc about LiveComposite
Window show None should not use config.window_show_transition.

And similary for window hide None and config.window_hide_transition.

Fixes #381.
Prompt the user to back up their project.

Brought to you by citizens against data loss.
play sound / queue sound should respect loop parameters.

Fixes #378.
sl2: Only pass scope to a displayable it it uses the scope.