improve docstring and add test fot to_rgb(<float>)

Even if the docstring was saing it, emphasis that to_rgb/to_rgba do not
accept float as input, but string representation of a float.

As one might be tempted to had float->gray conversion add failing test
in case the functionality is added.

Side fixes :
    change tuple([value]*3) to (value,)*3 for speed

In [18]: %timeit  tuple([0.4]*3)
1000000 loops, best of 3: 505 ns per loop

In [19]: %timeit  (0.4,)*3
10000000 loops, best of 3: 23.5 ns per loop

In [20]:  (0.4,)*3 == tuple([0.4]*3)
Out[20]: True

Closes gh-2609
qt4 subplottool is modal now + new layout
subplottool -> whats_new.rst
fix wrong backend name in whats_new
cleanup in subplottool + new style qt fix

Signed-off-by: Thomas Hisch <t.hisch@gmail.com>
setupext: put pkg-config -I, -L, -l locations at the head of the list

The assumption here is that we want the pkg-config info to take
precedence, so its -I and -L output should be prepended, not
appended to the lists used to construct the compiler and linker
commands.  It is less clear whether the -l info also should
be prepended, as it is in the changeset; ideally, it wouldn't
matter, but for some compilers, it might.  If so, -l can
be handled separately from -I and -L.
fix typo in furmsubplottool

Signed-off-by: Thomas Hisch <t.hisch@gmail.com>
minor pep8 to fix failing master builds.
Merge pull request #2626 from tacaswell/minor_pep8

minor pep8 to fix failing master builds.
Merge pull request #2610 from Carreau/close-2609

improve docstring and add test fot to_rgb(<float>)
Merge pull request #2623 from efiring/include_order

setupext: put pkg-config -I, -L, -l locations at the head of the list
Merge pull request #2594 from rhoef/newsubplottool

New layout for qt4 subplottool + QMainWindow -> QDialog