use g_spawn_command_line_async instead of execlp

@macie After merging #317 I noticed in the `execlp` and `execl` docs http://linux.die.net/man/3/execlp...

> The exec() family of functions replaces the current process image with a new process image

This didn't seem right to replace the `brackets` process with the browser. To avoid this, I moved to `g_spawn_command_line_async` https://developer.gnome.org/glib/2.34/glib-Spawning-Processes.html#g-spawn-command-line-async.

While cleaning that up, I also...

* Changed `CloseLiveBrowser` to also kill the same set of browsers google-chrome, chromium, and chromium-browser
* Fixed `ShowFolderInOSWindow` to use `file:` uris
* Use `g_get_user_config_dir` instead of `$HOME` to get the correct config folder, see https://github.com/adobe/brackets/issues/4932