Added tabindex value to links in the sidebar.

The tabindexes are reseted for each panel, beginning with 1. Because
of the accordion nature of the interface only the links of one of them
will be visible at any given time (the elementts with display:none
will not be focusable).

Change-Id: I3556a3f5b3b2db13c77ed6434b3f54ca7a530dc4
Closes-Bug: #1308442
Merge "Added tabindex value to links in the sidebar."