Release v0.5.6

Changes:

• FIX: KS dose not reflect the energy unit change
• FIX: rkCalc UI fails to add history.
• FIX: rclist.ui fails to show scrollbar.
• FIX?: rkcore contains many inadequate codes esp. for elastic reactions.
• child windows now inherit the font of the main window.
• the main window design changed to allow resizing.