- Prototyped the separation of editor and viewer functionality:
  * Added diagram's editor/viewer
  * separated editor and viewer for the markdown

- added search of content handlers by extension;
- forgot about code viewer

- These changes brought some troubles with embedded content and diagram menus
#122 Refactoring of framework
- dropped keep only one common instance for the markdown;
- added support of switch between edit and view modes
- started to think about cache: implemented simple cache on a format handler level
#122 - refactoring
- onFocus handler: notify format handler on focus enter (and exit)