Fixes#1978
* Move tail and non-tail handler to method
Defining them in the constructor is ugly.
* script: Iterate over defined actions instead of fixed list
* Separate running logic and lock m_output
* Include POLYBAR_FLAGS in linker flags
* Stop using m_prev in script_runner
* Join module threads in stop function
Joining in the destructor may lead to UB because the subclass is already
deconstructed but the threads may still require it to be around (e.g.
for calling any functions on the instance)
* Cleanup script module
* Update changelog
* Remove AfterReturn class
* Remove m_stopping from script module
* Fix polybar not reading the entire line from child process.
For every `readline` call we created a new fd_streambuf. This means once
`readline` returns, the streambuf is destructed and and pending data in
its temporary buffer discarded and we never actually read it.
* Remove unused includes
Since the forked processes are still our children, we need to wait on
them, otherwise they become zombie processes.
We now fork twice, let the first fork immediately return and wait on it.
This reparents the second fork, which runs the actual code, to the init
process which then collects it.
Ref #770
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
