Create a Settings window

[execveat]

Create a new Settings window, in the style of Burp's own Settings. The button should be placed on the right from the InQL tabs, just like Burp does it.

The window should expose normal settings and 'custom headers' functionality which in 4.x was available through it's own window.

I'd say in Burp Community version make all settings permanent. In Burp Pro there should be an option to change settings per project or globally.

Developing internal API for accessing persistence is tracked in #64

[execveat]

@matteoldani The config API is ready (https://github.com/doyensec/inql/blob/dev/python/inql/config/config.py), take from here.