Added note-area for displaying note contents
Define hsplit for displaying note titles + content

THe left side of the split contains a listbox of the titles of every
note in the database. The right side of the split will contain the
content of the note that is selected in the left side of the split (or
nothing if there is no note title selected on the left side).
Display hsplit in the main-frame
Added function callback for listbox selection

When an element in the listbox (the left side of the split containing
note titles) is selected, the contents of the note (stored as part of
the listbox element's model) are displayed in the text area on the right
side of the split.