Consider tab.index management

[joelpt]

• Option to make Sidewise mimic Chrome's ordering of tabs by observing tab.index
  • when Chrome creates new tabs
  • when user reorders Chrome tabs using drag&drop
• Option to reorder Chrome's tabs.indexes when moving rows in the page tree via drag&drop
• Option to coerce Chrome to use the same tab ordering that Sidewise naturally uses during tab creation/moving, e.g. when using "open selected links in new tabs"