Widgets/patterns sidebar to insert common diagrams in edit mode

[RazinShaikh]

Originally suggested in #178

Add a sidebar in edit mode where the user can add frequently used diagrams. Clicking (or dragging) a diagram should insert that diagram in the working window. Similar to custom rules, we can scan through a folder (configurable in settings) and add every .zxg file to the widgets sidebar. Alternatively, we should be able to add patterns to the sidebar like this: select part of the graph and right click->add.