UDIM: Show the UV grid even when images are loaded

Allow the UDIM grid to be shown and adjusted when there are images
loaded in UV edit mode. Right now the grid feature disappears once an
image is loaded and many have found this to be confusing.

Based on community and artist feedback, there was support to change this
behavior[1]

This patch does the following:
- Allows the grid to be shown even when images are present
- The max allowable dimensions for the grid has been increased from
10x10 to 10x100 to match the underlying maximum UDIM range that blender
supports

Note: This should not affect other Image editor modes like Paint/Mask or
the Render Result viewer etc. Future work in this area is currently
documented in a dedicated design task[2]

[1] https://devtalk.blender.org/t/the-udim-tile-grid-design-and-feedback-thread/20136
[2] https://developer.blender.org/T90913

Differential Revision: https://developer.blender.org/D11860

release/scripts/startup/bl_ui/space_image.py

@@ -1453,7 +1453,7 @@ class IMAGE_PT_udim_grid(Panel):
     def poll(cls, context):
         sima = context.space_data
 
-        return sima.show_uvedit and sima.image is None
+        return sima.show_uvedit
 
     def draw(self, context):
         layout = self.layout