Add Buffer Types

cc @jmatsushita

Allow Buffers to have types by introducing a new event type:

```haskell
data BufTypeChanged bufType = BufTypeChanged
```
`BufTypeChanged ReadOnlyBuffer` for read-only buffers in rasa-ext-files (rasa -R example.txt)
`BufTypeChanged (EditorBuffer languageType)` for specifying syntax choice in rasa-ext-syntax. Allowing to make syntax modular by having for instance a markdown syntax extension listen to `BufferTypeChanged (EditorBuffer MarkdownLanguage)`
` BufTypeChanged FileBrowser` for a file browser, etc.