Added option (in context menu) to display headers on a single line.

Added Compact Headers to messages opened in new windows.
Use onShutdown() instead of close() for uninstall & disable.
Added a workaround for a bug not hiding all headers in view all headers.
Added context menu to the main header area (view all/normal headers).
Dropped support for versions older than Thunderbird 78.

Added twisty.
Removed toolbarbutton.
Fixed uninstall & disable.
Added shortcut key [Alt-D].