Added proper implementation of the "ShowFolderInOSWindow" function for Linux. The target file is being highlighted, and "Show Extension Folder" (still) works.
For the highlighting to work, I had to detect the default file manager on the system, then run it with the target file as a parameter. If we failed to automatically determine the default file manager, the fallback behavior is to just open the directory containing the file (without the file being highlighted).
This is for https://trello.com/c/RF1ddQGK/898-linux-show-in-os-for-selected-file
Proper fix for "Show in OS" for Linux.

Now using a IPC call to FileManager1, similar to what Firefox does.
The fallback remains the same. That is - call gtk_show_uri() on the file's parent directory (probably not needed, but just in case). Also, fixed the URI generation.

Also, updated the appshell.gyp file to include the new compilation/linking flags needed for the new function to compile properly.
The library "libdbus-glib-1-dev" is now a dependency, so it should be added to the wiki page.
Clean up some comments.
Rewrote the DBus method call code to use the new APIs.

The GLib library now includes its own DBus bindings, and so they recommend switching to the new bindings for new applications.
This requires glib-2.0 version at least 2.26 (at the moment of writing the latest version is 2.40).
Removed the dependency of the binding library (libdbus-glib-1), updated appshell.gyp accordingly.
Merge pull request #477 from radorodopski/linux_show_file

Proper fix for "Show in OS" for Linux.