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/
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# Options
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option(
'nls',
type : 'boolean', value : true,
description : 'Install translation files'
)
option(
'doc',
type : 'boolean', value : true, yield : true,
description : 'build documentation with gtk-doc'
)
option(
'help2man',
type : 'boolean', value : true,
description : 'Whether or not to build man pages from --help output'
)
option(
'install-demo',
type : 'boolean', value : true,
description : 'Whether or not to install demo application'
)
option(
'introspection',
type : 'boolean', value : true, yield : true,
description : 'build gobject-introspection support'
)
option(
'tests',
type : 'boolean', value : true,
description : 'run unit tests'
)
option(
'vapi',
type : 'boolean', value : true,
description : 'Whether or not to build vapi files for gplugin'
)