Bugfix in FiveStarRatingWidget calculation, made a value const
BMView dialog look
Introduced FilePreviewHandler
Introduce LocalHTMLPreviewHandler
Added FileCategory notion to preview handlers
Some doc never harms
Added FilePreviewHandler managing in FileViewManager
Introduced FilePreviewerWidget
FileViewManager now previews using FilePreviewerWidget

Also FileViewManager::HasPreviewHandler now works according to the added
FilePreviewHandlers.
This revision compiles.
Automatic adding of all file handlers to FileViewManager
BMViewDlg previews default file
Tightened requirements of FileViewManager preview/edit/open functions
Now can preview HTML files correctly.