ComboBox: add align option

[pixelzoom]

ComboBox items are left aligned, with no option to do otherwise.

When the items are numbers, it would better to right align them. For example, here's how the Collision Counter currently looks in Gas Properties:

Add align option, with values 'left' (default), 'right', 'center'.

align should apply to both the popup list and the current value displayed on the button.

[pixelzoom]

This will also fix the alignment of temperature choices in Gas Properties. This combo box is currently composed of NumberDisplay items. The NumberDisplay items have align: 'right', but they have different widths, so the items are not right aligned in the popup list.

[pixelzoom]

The align option was added in the above commits, and align: 'right' is used in Gas Properties. What I thought was going to be trivial was downright scary — ComboBox has gotten complicated.

I tested with ?a11y and the focus highlight still looks correct:

Randomly assigned to @jessegreenberg to review, which works out nicely since my primary concern is breaking a11y.

[jessegreenberg]

Changes look good @pixelzoom. I tested a few other comboboxes, the option worked as expected, focus highlights were correct and screen reader output was not changed.

Closing.

[pixelzoom]

Reopening. This feature was broken during #430.

[pixelzoom]

Alignment was fixed in b47526a. Will continue further work in #430. Closing.