Add reverse support to category scale

[nagix]

This PR adds ticks.reverse support to category scale as well as the refactoring discussed in #6323 (comment).

This PR: https://jsfiddle.net/nagix/at8vfh6x/

Closes #3306

[nagix]

Added a boundary check at the end of getValueForPixel.

[kurkle]

Just FYI, I did something similar the other day. Handled the reverse thing in core.scale. Its based on layout refactor, so can't PR yet (also not near computer for weekend)

[benmccann]

you may also be able to use the new method in scale.linear/logarithmic.getPixelForValue/getValueForPixel

[benmccann]

can we either make this not necessary or add a comment explaining why it's needed? it seems redundant to me that we would store ticks in both ticks and _tickValues

[nagix]

Comment added. The labels need to be stored because they can be modified by the user callback while getPixelForValue requires the original label value.

[nagix]

getPixelForValue/getValueForPixel for logarithmic scale were refactored. I tried to refactor those methods for linear scale, but didn't include it because the size increased.

[nagix]

@kurkle I'm happy to review your code once the layout refactoring completes.

[DevidCIC]

Any updates on this ?
I see that if the Y-Axis consists of numbers then setting the reversed flags works. But if the Y-Axis consists of letters than the reversed does not work.