Add a ViewOption for an order of motors for wa macro

[daneos]

One of the ideas from Sardana Workshop at Solaris.

It'd be nice to have a configurable order in which wa macro displays motors. It could have the following values:

• alphabetic
• a list of explicit motor aliases
• a list of instruments
• a list of regular expressions to be matched

[cpascual]

I suggest to try to implement this using the same mechanisms (and syntax) as with the key argument for the sorted python function: https://wiki.python.org/moin/HowTo/Sorting

(I mean for the more generic cases)

[daneos]

I think the more generic solution, the better. I also have an additional request from our scientists, to allow empty spaces in motor list. They want wa to display motors in a way where each line of motors is different section of a beamline, and sections have different number of motors.