Rewrite the UI

[joelspadin]

The current UI code is a mess. I need to fix that.

The new UI should include:

• dropdown arrow next to the save button to make other saving options more discoverable
• A "Save tab to group" button on each group
• An overlay on the right edge that indicates where the mouse should be to trigger tab grouping.
• The hover effect to indicate grouping should be more obvious—probably a border surrounding the two items to be grouped.
• The effect when saving a tab to a group should be more obvious. The border could flash multiple times instead of once.