Redesign notebook folder structure.
Fix remove notebook issue reported by @atorok.
 Add support for autoSaveHtml

 Possibility to auto save html each time the md is saved.
 Usefull if one wants to sync and expose in a browser
If autoSaveHtml==True, edited notes will be autosaved to html/notes/
folder.
Export to html can be complete html (with css) or only html.
Autosaved html is html only, they share a css file.
Refactor mikiwindow.py: split __init__ into setup routines.
Fix drag/drop issue in notesTree
Adding attachment dockWidget
Add contextMenu to AttachmentView, correct select/deselect behavior.
Copy/paste images links to notesEdit will copy the actual image file to
attachment directory.
Relate notesTree actions to attachmentView behavior.
Add attachmentImage and attachmentDocument configuration.
Drag image/pdf from browser to mikidwon will download the file to
attachment folder.
Change insertImage to insertAttachment
Copy image from browser to mikidown works now.
Refactor out LineEditDialog to be shared by notesTree and notesEdit.
Add mikidown.generator module - a static site generator for mikidown.
Fix startup issue.
Add subcommand. Use `mikidown generate` to generate a static html site
from current directory.
Remove autoSaveHtml feature, as it doesn't handle subpages and attachments
well. Will add `mikidown generate --autoreload`.