Pause and Resume Torrents

Pausing and resuming torrents now works.
I took the liberty of changing the icon names according to the [standard](https://developer.gnome.org/icon-naming-spec/).
Yeah but I think it could be useful inside of the class, because of the event handlers in GtkTorrentTreeView needed to implement context menus can't call it while GtkTorrentTreeView could.
At this point it would just duplicate existing code.