Once an applet is opened, activate adjacent applets without clicking

[WatchMkr]

On current Pop!_OS, users can activate a menu in the top panel, then move the cursor to the left or right which will then activate adjacent menus without clicking them. We'll need the same feature in cosmic-panel to make browsing menus easier.

[WatchMkr]

Buttons like Workspaces and Applications are not automatically activated on-hover so we'll need some distinction in the applets that shouldn't activate.