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