LOKDocView: add part name retrieval.

Change-Id: Iac31de0410d7f6187e5884152c329375047a23f4
gtktiledviewer: show part names in selector.

Change-Id: I02ae5b729f96e36b3f226218b32495907cb63cf3
Impress: Implement getPartName.

Change-Id: I614838505fef901fc37e8d3906bb59c60b034453
ITiledRenderable: provide default implementations for some methods.

Writer doesn't understand the concept of parts at the moment, it
makes most sense to keep these dummy implementations central.

Change-Id: Iafbd89864b753ba2bed28a05b0f59df85f364feb
Impress/Tiled Rendering: return correct number for getPart.

It looks like page ids begin with 1.

Change-Id: I367285b8cfa1fcc9b8f22a9bb3c679d7e9579099
LOKDocView: add set_part_mode.

Change-Id: I0e14a535124e93979ef05431255911245a11aa29
Impress/Tiled Rendering: implement setPartMode.

Change-Id: I831953ac3fa1d0a15b64aa008cc3bfb79e72575a
Protect setPartMode and ensure we actually show the new mode.

Change-Id: I3642702c5bf268f3c9ab04162166078052804270
gtktiledviewer: add part mode selector.

Change-Id: Ia1e78df1d833f9b0fbda0b78136590c9c960fa27