Add auto detection of indent style

Submitted by Jeremy Bicha @jbicha

Link to original bug (#772396)

Description

Here's an idea for a new feature: smart indenting.

Since GNOME projects use a variety of indent schemes, it would be nice if GNOME Builder could be set to automatically detect the indent style used in a file and set automatic indentation to match the style already in use.

Because new commits to a file should obviously match the indent style already in use.