initialpose(or publishing PoseWithCovarianceStamped from canvas) support

[jihoonl]

PRs will follow to

• support PoseWithCovariance type visualization
• improve mouse handler
• update occupancy grid client
• add PoseWithCovariancePublisher
• add initial pose publisher example

[amy-fang]

Does the PoseWithCovariancePublisher finished? Do you have initial pose publisher example? I want to do “2D Pose Estimate” in my website.