wip: Add state saving plumbing
This branch is an exploration of apis to do save and restore application state.
Problems with the prototype:
We need save-id's on all parents of a widget, since we're recursing the tree. Some of those parents may be entirely internal and hidden from app developers, so it is impractical to expect them to set those ids.
A bit awkward to replace ::activate with ::create-window like this
In the exampleapp testcase, using the x to close the window doesn't save state, since we don't call g_application_quit() in that case
We need compositor support to save window positions on Wayland