Add ability to suppress blur on all focusable objects in a focus context

When a view receives a 'focusout' event, we relay that to the model by
calling ::blur. This is great for when users initiate the change in
focus, but 'focusout' events can *also* be caused by elements being
temporarily detached from the DOM.

The ::suppressBlur method gives the ability to ignore blur calls during
a certain operation. This is helpful, for example, when we want to
detach a model and reattach it somewhere else without changing its focus
state.

src/focus-context.coffee

@@ -3,3 +3,13 @@
 module.exports =
 class FocusContext extends Model
   @property 'focusedObject', null
+
+  blurSuppressionCounter: 0
+
+  isBlurSuppressed: ->
+    @blurSuppressionCounter > 0
+
+  suppressBlur: (fn) ->
+    @blurSuppressionCounter++
+    fn()
+    @blurSuppressionCounter--

src/focusable.coffee

@@ -12,8 +12,17 @@ class Focusable extends Mixin
 
   focus: ->
     throw new Error("Object must be assigned a focusContext to be focus") unless @focusContext
-    @focusContext.focusedObject = this
+    unless @focused
+      @suppressBlur =>
+        @focusContext.focusedObject = this
 
   blur: ->
     throw new Error("Object must be assigned a focusContext to be blurred") unless @focusContext
-    @focusContext.focusedObject = null if @focused
+    if @focused and not @focusContext.isBlurSuppressed()
+      @focusContext.focusedObject = null
+
+  suppressBlur: (fn) ->
+    if @focusContext?
+      @focusContext.suppressBlur(fn)
+    else
+      fn()