talkatu/talkatu

Simplify the buffers by making everything work on a TalkatuBuffer

The only things the content type specific buffers were doing now as defining
which formatting actions would be enabled. So we now created an flags and a
property on TalkatuActionGroup to fix that. By doing so, we are able to get
rid of TalkatuHTMLBuffer, TalkatuMarkdownBuffer, and TalkatuWholeBuffer.

Testing Done:
Ran the demo, tested all the buffers as well as the insert html and mark down actions.

Reviewed at https://reviews.imfreedom.org/r/2224/
demo/data/demo.ui
demo/talkatudemo.c
demo/talkatudemowindow.c
talkatu/data/attachmentdialog.ui
talkatu/data/attachmentpreview.ui
talkatu/data/editor.ui
talkatu/data/historyrow.ui
talkatu/data/history.ui
talkatu/data/input.ui
talkatu/data/linkdialog.ui
talkatu/data/toolbar.ui
talkatu/data/typinglabel.ui
talkatu/data/view.ui
talkatu/talkatuactiongroup.c
talkatu/talkatuattachment.c
talkatu/talkatuattachmentdialog.c
talkatu/talkatuattachmentpreview.c
talkatu/talkatuautoscroller.c
talkatu/talkatubuffer.c
talkatu/talkatucodeset.c
talkatu/talkatucore.c
talkatu/talkatueditor.c
talkatu/talkatuhistoryrow.c
talkatu/talkatuhistory.c
talkatu/talkatuhtmlbuffer.c
talkatu/talkatuhtmlrenderer.c
talkatu/talkatuinput.c
talkatu/talkatulinkdialog.c
talkatu/talkatumarkdownbuffer.c
talkatu/talkatumarkup.c
talkatu/talkatumessage.c
talkatu/talkatusimpleattachment.c
talkatu/talkatutag.c
talkatu/talkatutagtable.c
talkatu/talkatutoolbar.c
talkatu/talkatutypinglabel.c
talkatu/talkatuview.c
talkatu/talkatuwholebuffer.c
talkatu/tests/talkatutestactiongroup.c
talkatu/tests/talkatutesthtmlserialization.c