Middle Click Improvments

[bbb651]

• Middle click actions like closing a tab should be on mouse up instead of mouse down to be consistent with other actions
• Middle clicking the empty area in the tab area should create a new empty file like double clicking (matches firefox behavior)
• Middle clicking on file headers in mutlibuffers should open the file in a new tab (and specifically not trigger primary paste on linux)
• Middle clicking on a file in the project panel should open the file in a non-transient state
• Middle clicking on a notification should dismiss it

Things that match vscode behavior are marked with

cc @notpeter for quality week