Add support for generating QtCreator projects from GN.

This adds a new command line argument "--ide=" value to "gn gen"
which, when specified, generates a QtCreator project.

QtCreator is a quite powerful general-purpose (despite Qt in the name)
IDE when developing on Linux system with code completion and navigation.

Some interest in it has been demonstrated in the following thread:
https://groups.google.com/a/chromium.org/forum/#!topic/gn-dev/9U4_ytjrah8

BUG=

Review-Url: https://codereview.chromium.org/1883093002
Cr-Commit-Position: refs/heads/master@{#393514}
7 files changed