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
