Reading old project files may give projects without main plot collection
and well plot collection. Those are now created in the command feature
for adding a new well log plot, if needed.
Added command feature for adding new well log plots (right-click "Plots"
in the project tree view). Will create dummy Qwt widget added as an MDI
window.
TODO: Complete the relations between the entities, and add
properties/property editors. Create more advanced viewer widget for
plotting of multiple traces with multiple curves. Harmonize MDI stuff
with 3D viewers.
