Add support for working​ with meson projects

Qt Creator should have functionality for loading and working with meson based projects, similar to the functionality provided by the current CMake integration.

In other words, at least the following should be possible:

• Load a meson based project into Qt Creator.
• All source files should be properly indexed, using the correct build flags and include directories.
• Meson build files should be indexed and editable.
• Whenever a meson build description is modified, meson should be run and the index should be rebuilt.

For integrating with meson, the recommended solution is to use the IDE "API": http://mesonbuild.com/IDE-integration.html