Bugfix: Pan with Middle Mouse doesn't work on windows.

Description of fix

Added Middle Mouse entry into Stax keymap.

How to test it

• On windows, go into Adv. Drawing
• Pan the view with Middle Mouse Button

Checklist

Please check if your MR fulfills the following requirements:

• Build was run locally and any changes were pushed
• Lint has passed locally without error
• CHANGELOG.md has been updated
• Branch has been rebased on merge branch

FYI: @awesome_dev_too