Remove oversized margins in the AnimationTree editor

(cherry picked from commit 4f00ac7003c8818269b26b1acd1ce8ef308a8c08)

editor/plugins/animation_tree_editor_plugin.cpp

@@ -244,7 +244,7 @@ AnimationTreeEditor::AnimationTreeEditor() {
 
 	current_root = 0;
 	singleton = this;
-	editor_base = memnew(PanelContainer);
+	editor_base = memnew(MarginContainer);
 	editor_base->set_v_size_flags(SIZE_EXPAND_FILL);
 	add_child(editor_base);
 

editor/plugins/animation_tree_editor_plugin.h

@@ -56,7 +56,7 @@ class AnimationTreeEditor : public VBoxContainer {
 	HBoxContainer *path_hb;
 
 	AnimationTree *tree;
-	PanelContainer *editor_base;
+	MarginContainer *editor_base;
 
 	Vector<String> button_path;
 	Vector<String> edited_path;