Instead of PID, use PID file.

**Is your feature request related to a problem? Please describe.**
I have a project that before executing the main executable I need to run a few commands to setup the hardware. The easiest way for me to debug my main executable is to attach to it as it runs using the pid... But it is a hassle since the pid changes with every start.

**Describe the solution you'd like**
Every time my program starts I write a pid file. For example: /tmp/mainexec.pid.
If I could just specify this file in gdbgui instead of the PID, it would help a lot.

**Describe alternatives you've considered**
ps ax | grep "main exec name"

Start by locating the PID entry and attach-to-process flow in the gdbgui frontend and its backend communication path. Trace how the current PID value reaches the debugger, then verify that entering a PID file selects the process identified by that file and that the existing PID flow still works.

Written by the indexing model from the issue text.