[API] Add sidebar widget

attached is beta sidebar widget with working example.

to test example, click remove and see root is invisible, click add and see it is visible again.

this is my way of emailing you btw

I don't think the custom cellrenderer is needed, a GtkTreeView has built in system to make item trees. These files are using tabs, but we would prefer spaces ;) See bug https://bugs.launchpad.net/granite/+bug/818244

Could you also add the good gtk css class for the theming? get_style_context().add_class("sidebar") should be sufficient :)

Oh, and (it is not really important, but...) maybe you could change the signal names? I think something like sidebar_selection_changed and sidebar_item_clicked would be more intuitive...

Otherwise, it looks fine, it is really needed to keep a consistent layout between apps :)

1. the custom cellrenderer IS necessary in order to properly display the expander item
2. spaces... but why???
3. i was under the impression that "this.name = "SidebarContent";" is what added the styling. at least that's what i use in beatbox. or is this something new in gtk3?
4. as far as the signal names go, feel free to rename them whatever you want but do make sure you understand their context. item_clicked would not be appropiate imo. clickable_clicked happens when the clickable item on the treeiter (such as the eject button) is clicked. not when an item is clicked...

1 Ah ok, I forgot that the expander had to be at right, sorry.
2 A council decision
3 Ah, maybe, currently, marlin sidebar, for instance, uses the sidebar class, so...
4 ok, it is not so important, we'll add good documentation, it should be sufficient for general understanding

So, if someone wants to put that in a granite branch, make a sample in the demo apps and check the api is usable in most of our apps, it would be cool :)

It seems to me that BeatBox's sidebar is mature enough to be included in Granite. It would save hundreds of lines of code to new developers and is a good implementation. This would also improve consistency across applications.

Tom and I drafted a very complete sidebar API not so long ago, and it's been fully implemented. Please check out the linked branch :)