gcylc updater: change title bar by idle_add.

[hjoliver]

Close #1750.

Turns out this "idle" speculation was correct: #1750 (comment)

git blame suggests the problem was introduced first in Dec 2015, by f3e65f4

@matthewrmshin - please review/assign.

I'd like to get this out quickly in the next minor release as it has been a somewhat maddening problem!

[hjoliver]

A test case on my Ubuntu 14.04 VM;

[scheduling]
    initial cycle point = 2010
    [[dependencies]]
        [[[P1D]]]
            graph = foo => bar

• Start this suite as "test" and let it run.
• Repeatedly run gcylc test& (and close each gcylc if it works, else kill it).

On master, out of 12 attempts I got 6 successful starts, 4 blank locked-up windows that had to be killed, and 2 instant aborts with cylc-gui: Fatal IO error 11 (Resource temporarily unavailable) on X server :0..

On this branch, I get no problems.

[matthewrmshin]

The pep8 test is failing in my environment, wanting a bit more indentation here.

[matthewrmshin]

@benfitzpatrick please sanity check, but don't merge until the PEP8 issue is fixed.

[hjoliver]

pep8 fixed

[matthewrmshin]

@benfitzpatrick This should now be good to go. Please sanity check.