gdb/python/guile: check if styling is disabled in Color.escape_sequence

I noticed that the gdb.Color.escape_sequence() method would produce an
escape sequence even when styling is disabled.

I think this is the wrong choice.  Ideally, when styling is
disabled (e.g. with 'set style enabled off'), GDB should not be
producing styled output.

If a GDB extension is using gdb.Color to apply styling to the output,
then currently, the extension should be checking 'show style enabled'
any time Color.escape_sequence() is used.  This means lots of code
duplication, and the possibility that some locations will be missed,
which means disabling styling no longer does what it says.

I propose that Color.escape_sequence() should return the empty string
if styling is disabled.  A Python extension can then do:

  python
  c_none = gdb.Color('none')
  c_red = gdb.Color('red')
  print(c_red.escape_sequence(True)
        + "Text in red."
        + c_none.escape_sequence(True))
  end

If styling is enable this will print some red text.  And if styling is
disabled, then it will print text in the terminal's default color.

I have applied the same fix to the guile API.

I have extended the tests to cover this case.

Approved-By: Tom Tromey <tom@tromey.com>

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