[WIP] Added a new plugin, RSort. Sorts in decending order.

[MBertrand5470]

• This new RSort plugin does the reverse sort of the Sort plugin.
• Use CTRL-F10 shortcut to run the plugin.
• Modified po files for this new plugin.
• Note: only English and French files were completed, for other
  languages, only the translation squeleton was added.

[MBertrand5470]

Fixes issue #501 .
Pull request #503 was cancelled, this new one adds plugin RSort.

[mtwebster]

Hi,

While I appreciate the effort here, it's not necessary for this to be a new plugin - it can just be added to the existing sort plugin.

You simply add your action entry to the array in the other plugin:

static const GtkActionEntry action_entries[] =
{
    { "Sort",
      NULL,
      N_("S_ort lines"),
      "F10",
      N_("Sort the current document or selection"),
      G_CALLBACK (sort_cb)
    },
    { "RSort",
    ...
    ...
    ...
    }
};

and then add your rsort_cb() function. Then do whatever you need to reverse the sort depending on the action (this may be as simple as altering the behavior of compare_line, not sure).

Please don't include changes to the .po files, we handle that on launchpad.