User config of tweening & transitions. Fixed lost undo history. Focus and progress indicators

• Added display of progress indicator when rendering of graph takes longer than 800 ms. #38
• Added indication of focused pane by increasing its elevation, thereby making it cast more shadow. #39
• Fixed lost undo/redo history when node or edge format drawer is opened. #53
• Added user configuration of transition duration. #52
• Added user configuration of tweening precision. #44
• Added user configuration of path & shape tweening enable/disable. #43