Branch tag gdb_7_6-branch added for insight specific sources

I've added the gdb_7_6-branch branch tag for the following insight
specific source directories:

    gdb/gdbtk
    gdb/testsuite/gdb.gdbtk
    tcl
    tk
    itcl
    libgui

This time around, I used the -D 2013-04-10 as the branch point for
the insight specific sources.  I didn't add a tag for this branch
point, though I can do so if requested.

Kevin