explicit parent mechanism for CCoverDropArea
add 'fold' and 'unfold' icons
mini-view mode for song-header-editor
stretch diagram area
move display statusbar/toolbar from application menu to preferences
sizeHint methods are virtual
wrapper methods for CDiagramWidget
diagram-area: provide read-only mode

In this mode:
 - the addNewDiagram button is hidden
 - diagrams can neither be edited nor removed
diagram-area: rework selection mechanism for diagrams

 - in read only mode: multiple selection is allowed through Ctrl modifier
 - else, single selection
diagram-area: support specifying a number of columns
diagram-area: add filter capabilities
diagram-area: support separators

Separators are bold labels that stretch over a whole line in the grid layout. For instance, they can correspond to the \chordname macro in the songbook's chords.tex file.
diagram-area: use CDiagramWidget class instead of CDiagram
minor corrections
diagram-editor: add new diagramArea to the dialog
diagram-editor: add reset capability
diagram-editor: set size hint
diagram-editor: rename slot
diagram-editor: align diagram label
diagrams: don't emit changed signal unless the content has changed