Move serialization of active pane from Pane to PaneContainer

Fixes #2694
Fixel #2853

Previously, we were storing an `active` boolean for each pane. We've
had some strange bugs where every pane is serializing `active: false`,
which causes exceptions when loading up the stored data.

This new approach serializes the activePaneId on the PaneContainer
itself. Since the PaneContainer is the source of truth regarding the
active pane, it makes more sense to handle it here.

This unfortunately changes the serialization version for the
PaneContainer, so people won't have their state persisted after
upgrading. But it seems better than leaving cruft to handle the old
serialization situation.

src/pane-container.coffee

@@ -8,6 +8,8 @@ class PaneContainer extends Model
   atom.deserializers.add(this)
   Serializable.includeInto(this)
 
+  @version: 1
+
   @properties
     root: -> new Pane
     activePane: null
@@ -27,10 +29,12 @@ class PaneContainer extends Model
   deserializeParams: (params) ->
     params.root = atom.deserializers.deserialize(params.root, container: this)
     params.destroyEmptyPanes = atom.config.get('core.destroyEmptyPanes')
+    params.activePane = params.root.getPanes().find (pane) -> pane.id is params.activePaneId
     params
 
   serializeParams: (params) ->
     root: @root?.serialize()
+    activePaneId: @activePane.id
 
   replaceChild: (oldChild, newChild) ->
     throw new Error("Replacing non-existent child") if oldChild isnt @root

src/pane.coffee

@@ -40,14 +40,12 @@ class Pane extends Model
     @subscribe @items.onRemoval (item, index) =>
       @unsubscribe item if typeof item.on is 'function'
 
-    @activate() if params?.active
-
   # Called by the Serializable mixin during serialization.
   serializeParams: ->
+    id: @id
     items: compact(@items.map((item) -> item.serialize?()))
     activeItemUri: @activeItem?.getUri?()
     focused: @focused
-    active: @active
 
   # Called by the Serializable mixin during deserialization.
   deserializeParams: (params) ->