When given a file, open its folder and select it

A file can now be opened directly (e.g. on the command line) - the
folder it's in is opened like normal, but the file is automatically
selected in the list after the folder loads.
Another code style change

Make all arguments vertically aligned. This looks nicer IMO. Also, use
'//' comments for the copyright notice at the beginning of files.
Nitpick several function/variable names

No real functional changes, except using Glib::RefPtr<Gio::Menu>::
cast_dynamic(), since Builder stuff seems to use dynamic casts.
Add more comments and nitpick existing ones

Comb through and add comments that were missing, and reword some
existing ones. Use backticks (`) to refer to functions, variables, etc
in comments.