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/
# Building
Talkatu uses [meson](https://mesonbuild.com) as it's build system.
Meson requires you to build in a directory other than the source directory.
To compile you can use the following commands:
```
meson build
cd build
```
To test Talkatu without installing, you can run the following command from the
build directory.
```
meson devenv talkatu-demo
```
To install you can use `ninja install` from the build directory.