Add "cycle orders"

[edam]

Add configuration option to select the order in which the window positions are cycled.

Perhaps more consideration should be given to the options available. But I have added only 2:

• default, which is selected by default and which cycles positions in the previous order, and
• small_first, which starts with the smallest position first.

[coveralls]

Coverage decreased (-0.5%) to 60.502% when pulling ab13a1d on edam:add-cycle-orders into 647eaf0 on ssokolow:master.

[ssokolow]

First, I want to thank you for putting the effort into this.

That said, I have two concerns:

1. Now that I'm working on QuickTile again, coming up with a general solution for customizing cycling behaviour is a high priority.

   I anticipate this mechanism existing for only a short period of time and, because I try to maintain backwards compatibility for configuration files, it would leave behind added complexity for the config migration code effectively forever, despite existing for such a short period. (and I still need to get around to going back through old versions and generating a bunch of test files to ensure the migrations actually work.)

2. My first goal is to finish refactoring the code and de-snarling __main__.py is likely going to be my next target. If I haven't merged this first, the PR will break.

(In short, this is a textbook example of why the developer's guide asks people to open feature requests before pull requests so we can coordinate on things before effort is put in.)