Add a setting to automatically ask gdb for pwd on executable start

[aolo2]

What the title says. I find myself hitting the button every time after I step into an executable.

[nakst]

I've made the following changes in the latest commit:

• Preset commands can now run the last command asynchronously, by adding a & at the end.
• Preset commands can now run special commands.
• Keyboard shortcuts can now run special commands.
• I added a special command to get the working directory from GDB, called gf-get-pwd.
• I added a special command to run a preset command, called gf-command.
• Standard keyboard shortcuts can be overridden.

Putting this all together, you can add a new preset command to configuration file:

[commands]
run-pwd=start;gf-get-pwd;c&

This starts the application paused, get the working directory from GDB, and finally continues execution asynchronously.

And then you can overwrite the standard keyboard shortcut for running the application to call into this preset command instead:

[shortcuts]
Shift+F5=gf-command run-pwd

There have been several structural changes made in this commit so please tell me if something has been broken!