controller: Detach shell commands from polybar

Shell commands triggered from action tags used to block polybar until
they finished.

Since we are not actually interested in the output of the commands, it
makes sense to run them completely detached from polybar and have
polybar not block when executing these commands.

Now the spawned child processes no longer get killed when polybar
exits. This is fine because polybar is not responsible for these
processes since they were explicitly started by the user through click
commands.

Ref: #770
Ref: #1680

include/components/controller.hpp

@@ -20,8 +20,6 @@ POLYBAR_NS
 
 enum class alignment;
 class bar;
-template <output_policy>
-class command;
 class config;
 class connection;
 class inotify_watch;