Image support

• Added support for images within nodes.
• Added basic image editing support.
• Added support for dragging and dropping local and web images.
• Added support for resizing node width.
• Changed cursors when over a task button.
• Changed location of task and note elements in a node.
• Added support for keeping the map from scrolling off screen.
• Added support for shift-mousewheel to scroll horizontally.
• Added support for control-mousewheel to zoom in/out.
• Fixed issue with drawing background when zoom factor was small.
• Custom icons are now stored as a gresource rather than in the file system.
• Other minor bug fixes.