Modules that don't produce any output are hidden by the controller
(don't have margins or separators).
The tray module should also do that for `format = <tray>` when there are
no icons.

This required the visibility handling to be tied to the module
visibility instead of being handled by the renderer.
Otherwise, the renderer would hide the tray (because the %{Pt} tag was
never sent) and the tray would not unhide when new icons appeared; it
can't differentiate between hidden because empty and hidden because the
module is hidden by the user (the latter is the reason the renderer does
hiding at all).

Fixes #3036

No longer needed since tray visibility is now controlled directly by the
enclosing module

The 'value' variable that was used for validation, was empty because it
was used in a move at the beginning of the function.

Fixes #3043

Closes #3042
Includes polybar/i3ipcpp#16