Add tickAlpha setting to AxisItem style

In the AxisItem class, ticks are arranged in a hierarchy (major, minor, subminor). When you have a plot with gridlines, the higher ticks are drawn with heavier lines. I needed to produce a plot where the gridlines all look the same, but could not find a straightforward way of doing so. I could have subclassed the AxisItem class and used the custom class in my code, but I felt that other people might find this added functionality useful, so I instead created this pull request.