Improvements of state machine

- Open the menu to add new animation nodes by dragging the transitions to
empty areas and automatically connecting them.
- Adds box selection to the state machine.
- Add feature to group/ungroup selected nodes in a "sub" state machine.
- Add start/end node by default. In addition, add new color to these
nodes to differentiate then.
- Add tooltip for transitions to show the connection "from -> to".
- Add new "type" of transition line when multiple transitions are
grouped.
- Add popup to connect nodes in sub state machine.
- Add dialog to select which nodes can be deleted when they are grouped.
- Add classes:
	AnimationNodeStartState
	AnimationNodeEndState
	EditorAnimationMultiTransitionEdit
- Implements disabled transition

API Changes:
- Now it's posible to add transitions between state machines,
`AnimationNodeStateMachine::add_transition` will works with relative path,
this means you can use it like this `add_transition("Idle", "Walk", tr)`
or `add_transition("Idle", "StateMachine/Shoot)`.

doc/classes/AnimationNodeStateMachine.xml

@@ -39,12 +39,6 @@
 				Adds a transition between the given nodes.
 			</description>
 		</method>
-		<method name="get_end_node" qualifiers="const">
-			<return type="String" />
-			<description>
-				Returns the graph's end node.
-			</description>
-		</method>
 		<method name="get_graph_offset" qualifiers="const">
 			<return type="Vector2" />
 			<description>
@@ -72,12 +66,6 @@
 				Returns the given node's coordinates. Used for display in the editor.
 			</description>
 		</method>
-		<method name="get_start_node" qualifiers="const">
-			<return type="String" />
-			<description>
-				Returns the graph's end node.
-			</description>
-		</method>
 		<method name="get_transition" qualifiers="const">
 			<return type="AnimationNodeStateMachineTransition" />
 			<argument index="0" name="idx" type="int" />
@@ -157,13 +145,6 @@
 			<description>
 			</description>
 		</method>
-		<method name="set_end_node">
-			<return type="void" />
-			<argument index="0" name="name" type="StringName" />
-			<description>
-				Sets the given node as the graph end point.
-			</description>
-		</method>
 		<method name="set_graph_offset">
 			<return type="void" />
 			<argument index="0" name="offset" type="Vector2" />
@@ -179,12 +160,5 @@
 				Sets the node's coordinates. Used for display in the editor.
 			</description>
 		</method>
-		<method name="set_start_node">
-			<return type="void" />
-			<argument index="0" name="name" type="StringName" />
-			<description>
-				Sets the given node as the graph start point.
-			</description>
-		</method>
 	</methods>
 </class>