UI: preset popover buttons in panel headers.

Moves the preset into a menu for the panel header, so it can be changed
without opening the panel and takes up less space. Two remaining issues:

* For long lists the add new preset button can be scrolled off screen.
* We should support showing the name of the chosen preset in the panel
  header, but the current preset system does not support detecting which
  preset is used.

Differential Revision: https://developer.blender.org/D3366

release/scripts/startup/bl_ui/properties_physics_fluid.py

@@ -20,13 +20,14 @@
 import bpy
 from bpy.types import Panel, Menu
 from bpy.app.translations import pgettext_iface as iface_
+from bl_operators.presets import PresetMenu
 
 
-class FLUID_MT_presets(Menu):
+class FLUID_MT_presets(PresetMenu):
     bl_label = "Fluid Presets"
     preset_subdir = "fluid"
     preset_operator = "script.execute_preset"
-    draw = Menu.draw_preset
+    preset_add_operator = "fluid.preset_add"
 
 
 class PhysicButtonsPanel:
@@ -240,11 +241,7 @@ class PHYSICS_PT_domain_gravity(PhysicButtonsPanel, Panel):
             col.prop(fluid, "simulation_scale", text="Meters")
 
         col = split.column()
-        col.label(text="Viscosity Presets:")
-        sub = col.row(align=True)
-        sub.menu("FLUID_MT_presets", text=bpy.types.FLUID_MT_presets.bl_label)
-        sub.operator("fluid.preset_add", text="", icon='ZOOMIN')
-        sub.operator("fluid.preset_add", text="", icon='ZOOMOUT').remove_active = True
+        FLUID_MT_presets.draw_menu(col, text="Viscosity Presets")
 
         sub = col.column(align=True)
         sub.prop(fluid, "viscosity_base", text="Base")