Tab navigation for model window

[daemontus]

Create a tab-based navigation for the model editor window.

Expected functionality:

• A tab can be completely disabled (e.g. for a workflow that doesn't need it).
• A tab can be "inactive" if a particular feature is unused but enabled. E.g. if we are editing a sketch but haven't specified any properties/assertions yet, the tab is shown, but is not prominent (e.g. slightly transparent and to the side).
• A tab can be "detached" when it is visible in a separate window.
• A tab can be "locked" when it is visible even if another tab is selected. When multiple tabs are visible, they can be resized by dragging. Reordering tabs should also reorder the content.
• A tab should be able to display a badge with error/warning indicator.

See design/tabs.png.