Add score display support for shapes

[safa0]

Summary

This PR adds support for displaying scores/confidence values on shapes in the labelme annotation tool.

Changes

• Added _draw_score_text() method to the Shape class
• Shapes can now display text scores from other_data['score_text']
• Score text is rendered in the upper-left corner of the shape's bounding box
• Text has a semi-transparent white background for better readability
• Uses 10pt bold font with black text

Use Case

This feature is useful for:

• Displaying confidence scores from ML model predictions
• Showing detection scores when importing pre-labeled data
• Adding any numerical or text metadata to be displayed on shapes

Implementation Details

The score display:

• Only renders when score_text exists in the shape's other_data dictionary
• Positions text in upper-left corner with 3px padding
• Includes semi-transparent background (rgba: 255, 255, 255, 200)
• Scales properly with zoom level
• Restores original painter pen after drawing

Screenshots

The score text appears as a small label in the upper-left corner of annotated shapes when other_data['score_text'] is set.

[wkentaro]

we shouldn't read from other_data, so this has to be other way.

[safa0]

I first added a score specific field, but then thought we already have "other_data", what do you suggest? a specific "score" field per shape?

[wkentaro]

Probably steps would be like:

1. add a capability to draw label (or label + groupid)
2. add a dropdown to select what field to show (supports flags, other_data, etc)

?

I am for the direction, it clearly adds value.