Re-added use of g_spawn_command_line_sync in place of popen, to execute an external comment.

This effectively reverts commit 51b662b0
Rational:
- the failure of g_spawn_command_line_sync was a glib bug 3.6.0, that got fixed since then (3.6.2)
- the use of popen generates unnecessary console when compiled for windows
- the use of popen makes it difficult to redirect stderr, which results in error messages being printed
on screen when the executed command failed (for instance because the relevant application is not
installed.

CCBUG: 318891