Separate root-related code

This creates a root.c and moves bits and pieces from elsewhere into it.

* layout_init has been renamed to root_create and moved into root.c
* root_destroy has been created and is called on shutdown
* scratchpad code has been moved into root.c, because hidden scratchpad
containers are stored in the root struct

sway/meson.build

@@ -7,7 +7,6 @@ sway_sources = files(
 	'debug-tree.c',
 	'ipc-json.c',
 	'ipc-server.c',
-	'scratchpad.c',
 	'security.c',
 	'swaynag.c',
 
@@ -150,6 +149,7 @@ sway_sources = files(
 	'tree/arrange.c',
 	'tree/container.c',
 	'tree/layout.c',
+	'tree/root.c',
 	'tree/view.c',
 	'tree/workspace.c',
 	'tree/output.c',