stacks: Merge the whole stacks directory into one content_stack.

Window now handles the Show widget. Empty is now an unnamed stack child. ShowsView is now initialized delayed when it opens.

Closes #329 (closed)

• test
  • empty
  • empty after unsub
  • not empty after undo
  • home_view updates
• There are too many files in stacks that do nothing, but forward calls.
• Widgets are stored in Rc wrappers that are unnecessary.
• Shows_view shouldn't be initialized before it is actually visible to speedup the startup-time.
• At least one level of Stacks can be dropped and Empty can be added to the highest stack as unnamed page (make sure to test with keybindings and small toolbar).