Minor fixes

[giordano]

• Initialise members of NammuController in the constructor
• In get_viewport_carets test the type instead of catching all exceptions

These changes, together with #398, make the failure rate of tests on Travis to drop sensibly as far as I could see. #397 may not be needed in the end.

[giordano]

although it may be helpful to have some record of the problems that these changes fix

These are strange, random, and difficult to reproduce errors that I spotted in the last days. If I remember well, the one related with the view port probably mostly happens in a thread and doesn't even cause tests to fail.

I'll try to write a little bit more in the commit message, but don't expect too much.

[ageorgou]

As far as I remember, the AtfAreaView issue was that sometimes the code "decided" that the pane was split because of (presumably!) an exception from top_ch, bottom_ch = self.get_viewport_top_bottom(viewport).

The other was that some attributes of NammuController were not created in the constructor, but only when opening a file. This only came up when running a subset of the tests (the ones that call arabic()(?) without opening a file), because when running the whole suite, a file has already been opened by previous tests. I think it can't occur in normal usage, but it's cleaner to create the attributes in the constructor anyway.