UI: rename PresetMenu to PresetPanel, move to bl_ui.utils

Confusing to call a menu a panel when subclasses need to define
panel specific variables.

Avoid having bl_ui depend on bl_operator module too.
Since this isn't an operator, add utils modules for shared types.

release/scripts/startup/bl_ui/utils.py

@@ -0,0 +1,56 @@
+# ##### BEGIN GPL LICENSE BLOCK #####
+#
+#  This program is free software; you can redistribute it and/or
+#  modify it under the terms of the GNU General Public License
+#  as published by the Free Software Foundation; either version 2
+#  of the License, or (at your option) any later version.
+#
+#  This program is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU General Public License for more details.
+#
+#  You should have received a copy of the GNU General Public License
+#  along with this program; if not, write to the Free Software Foundation,
+#  Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+#
+# ##### END GPL LICENSE BLOCK #####
+
+# <pep8 compliant>
+
+from bpy.types import Menu
+
+
+# Panel mix-in class (don't register).
+class PresetPanel:
+    bl_space_type = 'PROPERTIES'
+    bl_region_type = 'HEADER'
+    bl_label = "Presets"
+    path_menu = Menu.path_menu
+
+    @classmethod
+    def draw_panel_header(cls, layout):
+        layout.emboss = 'NONE'
+        layout.popover(
+            panel=cls.__name__,
+            icon='PRESET',
+            text="",
+        )
+
+    @classmethod
+    def draw_menu(cls, layout, text=None):
+        if text is None:
+            text = cls.bl_label
+
+        layout.popover(
+            panel=cls.__name__,
+            icon='PRESET',
+            text=text,
+        )
+
+    def draw(self, context):
+        layout = self.layout
+        layout.emboss = 'PULLDOWN_MENU'
+        layout.operator_context = 'EXEC_DEFAULT'
+
+        Menu.draw_preset(self, context)