symbols: provide a bit more path information in the goto-symbol popup.

symbols: provide a bit more path information in the goto-symbol popup.

From #1069:
> At the moment if symbols of the same name are defined in identically named
> files, it's hard to distinguish which file is which because there's no path
> in the popup.

> The popup should show part of the path until a directory where the paths
> differ so it's possible to distinguish the different files. At the same time
> there should probably be some top limit for the length of the paths as they
> can make the popup too wide.

This addresses the above by showing more of the file's paths but still try
to make it as short as possible. The file list is processed by the new
utils_strv_shorten_file_list(), as a result the popup will list files with
the common prefix stripped and the longest common sub-path ellipsized.

As a result, the file list shows enough of the path to make them unique but
still is still very short and doesn't make the dialog too wide.

Fixes #1069.