Add the ability to run pre-configured actions

## Problem

There is no way to run pre-configured actions. So, every time you need to run your typical Gtk/Vala application — you need to run `ninja` and `./src/io.elementary.code` commands yourself.

## Proposal

Make an extension for running pre-configured actions in the integrated terminal. This would ease debugging because all you need to do to run a program is click on the big green media-playback-start button.

## Prior Art

I imagine having a media-playback-start button in the header bar, and right next to it, a combo box with named action to be executed. Users should add these runners by saving a string to be executed in the shell and naming it. Actions should run in the separate session of the integrated terminal, not the one you access by the "Terminal" extension.
As a reference, you can look at the Intellij IDEA's feature named [Run/debug configurations](https://www.jetbrains.com/help/idea/run-debug-configuration.html).