Update quick-links on clicking Mark/Unmark on popup.

Marking/unmarking now happens through regular functions in the
background page instead of through message passing.

All functions in the marking/unmarking chain now accept
callbacks. Quick-links are updated through a callback which takes care
of all the earlier timing issues.
Make C-. use new mark/unmark functions.
Increase preview size.
Show favicon for nodes if possible.
Update 'image' node type so tooltip is shown.
Update node when favicon is loaded.
Show a tree widget in the panel.
CSS changes for visualization.
Change shortcuts and show titles in quick-links.

Shortcuts are now Ctrl+Shift+<key near left Ctrl/Shift>.
The quick-link popup shows titles of visited pages if available, and
shows the URL on hovering over the title.
base.css has now been merged into hypertree.css.
Try switching to an open viz tab on Ctrl+Shift+V.
Use single triangle which is more compatible.