fixed empty database issue
selecting a heading in heading tree moves sheet to the heading
refactored button placement in toolbar
show/hide button for heading tree
init tagging tool
tagtree selection and tag creation
tag fixes
assigning tags works again
remove tags works again
query tags for multiple elements works
query tags for multiple elements for NoteElement class
Merge branch 'backend' into ui_gtk_gnome
model of UiNoteElements is accessible from extern now
tag tool shows correct tags
tag tool assigns correct tags