UI: Better center-align Properties search button in header

For some reason the layout code doesn't center the search button properly. Add
a blank icon button to add some padding, dynamically resized as the region size
changes. This is quite finicky and not at all perfect. But it makes the search
button look far less off-place.

release/scripts/startup/bl_ui/space_properties.py

@@ -26,11 +26,22 @@ class PROPERTIES_HT_header(Header):
     def draw(self, context):
         layout = self.layout
         view = context.space_data
+        region = context.region
+        ui_scale = context.preferences.system.ui_scale
 
         layout.template_header()
 
         layout.separator_spacer()
 
+        # The following is an ugly attempt to make the search button center-align better visually.
+        # A dummy icon is inserted that has to be scaled as the available width changes.
+        content_size_est = 160 * ui_scale
+        layout_scale = min(1, max(0, (region.width / content_size_est) - 1))
+        if layout_scale > 0:
+            row = layout.row()
+            row.scale_x = layout_scale
+            row.label(icon='BLANK1')
+
         layout.prop(view, "search_filter", icon='VIEWZOOM', text="")
 
         layout.separator_spacer()