Merge lp://staging/~waldner/terminator/plugin-ng into lp://staging/terminator/gtk3

Is this too invasive? Bad style? All comments welcome.

I like the idea of generalizing the handling of selected patterns and being able to extend it to show different kinds of context menus. Conceptually seems quite interesting. Style looks fine to me, in fact it refactors things to look a bit cleaner. Needs a better name (I don't have suggestions myself, sorry), and I would like to see a couple use case/example ideas at the top of the docs (i.e. what are some useful kinds of things people could expect do with this functionality). A test/example plugin should be included. Those three things fixed I'd give this a +1.

For an example plugin, see https://github.com/waldner/terminator-runwith-plugin (needs the plugin-ng patched terminator, of course). There are some (admittedly simplistic and a bit silly) usage examples there as well.

I'll think about the name (suggestions welcome). What I use it for is to extract patterns from the terminal text (regex can include context, later thrown away, to avoid false matches) and use them as arguments to an external program or command. Think: you have a filename, and want to open it with your editor of choice. You have a github URL, and you want to run "git clone" with it. (more to be added)
Right-click on the element, runwith -> yourcommand and you're done. I'll try to come up with more use cases. Perhaps it's not a popular thing to do, but it definitely helped me.

Probably the most notable use case is the ability to create true context-sensitive menus, which was plain impossible with the old plugin system.