Shared modules across bars

[Alexays]

Some modules doesn't need to be duplicate across bars.
e.g. cpu, memory, idle_inhibitor, brightness, clock, ...
Pb: module config can be different across bars.

Will fix #296

[b10n]

Custom modules do not (always) need to be duplicated across bars. I noticed that Waybar 0.9.9 creates a subprocess for each bar. Could this be made configurable?
Perhaps a separate issue is that a subprocess becomes a zombie when a bar is no longer shown, for instance when the output configuration changes.

[pocek]

#1276 addresses custom modules. Edit: Except it doesn't work the way I had hoped for. The execs are still not shared across bars.

[lilydjwg]

I don't want share clock widgets, because I want it to show seconds on ordinary displays but not on e-ink ones (Constantly changing content is annoying there).

There may also be some other cases sharing is not desired, e.g. different sizes (big monitors vs small ones) or colors (dark theme is terrible for e-ink displays). So sharing (and with which) should be configurable, e.g. define widgets without referring to an output, and then put their ids into each output's configuration.