Branch tag gdb_7_3-branch created for insight-specific sources

I've added the gdb_7_3-branch branch tag for the following insight
specific source directories:

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

When figuring out which date should be tagged, I tried using the same
date as that used by gdb, 2011-04-01, but that did not work due to
some changes that had been made to gdb, but for which insight had not
yet been changed to match.  I ended up using 2011-04-12.  Sources from
this date includes the requisite fixes so that gdb/gdbtk will build
without error.

Kevin