UI overhaul and window-by-window restoration.

NOTE: tabs saved with an older version of this extension can NOT be restored on the updated version. Please back up these tabs before updating. (Tabs are saved in the browsers local storage, look there for manual recovery)

Enhancements:
- "Saved!" Feedback on saving tabs
- CSS to make extension prettier
- Tabs are now grouped by window
- Individual windows can be restored (either to active window or new window)
- Links are shown as links not checkboxes

Multi-Window Support
Tabs are now restored in the same grouping as when saved. This way organization via windows in maintained when restoring.

e.g. If a user has 2 windows open such that Window 1 contains Tab A, Tab B, & Tab C and Window 2 contains Tab D, Tab E:
Clicking Save will result in a list containing all of the tabs.
After closing the browser and then reopening, the user will have one active window.
Clicking Restore All will result in 3 windows such that Window 1 contains the tabs the user already had opened, Window 2 contains Tab A, Tab B, & Tab C, and Window 3 contains Tab D and Tab E