[box plot] add X tick layout control (#6903)

superset/assets/src/explore/controlPanels/BoxPlot.js

@@ -35,7 +35,7 @@ export default {
       expanded: true,
       controlSetRows: [
         ['color_scheme'],
-        ['whisker_options'],
+        ['whisker_options', 'x_ticks_layout'],
       ],
     },
   ],

superset/data/world_bank.py

@@ -234,6 +234,7 @@ def load_world_bank_health_n_pop():
                 since='1960-01-01',
                 until='now',
                 whisker_options='Min/max (no outliers)',
+                x_ticks_layout='staggered',
                 viz_type='box_plot',
                 groupby=['region'])),
         Slice(