Meson plugin: First working version

Summary:
This patch implements the basic functionality of the meson plugin.
Introspection (targets, include dirs, etc.) is not implemented yet.

The current features are:
  - (re)configuring a meson project
  - building with ninja
  - GUI for configuring a build directory (currently only install prefix and build type)
  - managing multiple build directories
  - GUI for adding a new build directory

Reviewers: apol, arrowd

Reviewed By: arrowd

Subscribers: arrowd, rjvbb, flherne, rizzitello, yurchor, kde-doc-english, kdevelop-devel

Tags: #kdevelop, #documentation

Differential Revision: https://phabricator.kde.org/D17278