Rename "Back" to "Home" when applicable

[JeanMertz]

Right now, if you open a task, you can click the "Back" button to go back to the home screen. If you go from one task to another (f.e. by using #20), if you then click the "Back" button, you go back to the previous task you had open.

I propose changing the first situation to rename "Back" to "Home", the rest stays the same. This makes it a bit more clear if you're back at the "first" open task, or in a nested task (although we could also expose other ways to make this more clear).