Add onSomethingDown callbacks

[ericluap]

Adds callbacks for the different clicks on mouse down.

This is useful for certain things like changing the position of the cursor in an Input component on mouse down.

[glennsl]

Thanks for the PR @SirFurness! I'm not sure this is really needed on Clickable though. onMouseDown is already available on View, which is what you'd typically use to implement something like this. I could see the value of having onRightMouseDown for convenience if that was a really common need, but I don't think it is. Consequently this mostly just complicates the API I think. I'm certainly open for arguments to the contrary though.

[glennsl]

This should be onMouseDown, onRightMouseDown etc. to disambiguate it from for example onKeyDown.

[ericluap]

You're probably right that it's not a common need. I was mainly thinking in terms of future improvements to the Input component. It needs the cursor to change on left click mouse down and it will eventually need a word to be highlighted on double click mouse down.

I don't think any component should use onMouseDown from a View because it is called for any type of mouse button. This leads to things like the Slider component moving with a right-click drag. My current thinking is just that anything that cares about mouse clicks should use a Clickable so that things like onMouseDown work only with left clicks and code checking for what mouse button was used does not need to be duplicated. It could also make the intent more clear (e.g. that Slider is meant to be clicked).

I guess it depends on how the Clickable component is meant to be used. Maybe there should be a different component?

[glennsl]

I'm honestly not really sure what Clickable is supposed to be either, as the events it exposes could easily have been made available directly on View.

I don't think any component should use onMouseDown from a View because it is called for any type of mouse button. This leads to things like the Slider component moving with a right-click drag.

The mouse button is passed in the events args. That's how you're able to implement this. And this is analogous to how onMouseDown works on the Web. For something as rarely needed as this I personally think that's fine.

Left-only click, right click and double click is common enough that I think it warrants separate events, and they fit well with Clickable. But the rest I'm not so sure about design-wise. It would be nice to have a holistic idea and some overall guiding principles for where we're going with this I think.

[ericluap]

Sounds good! A document with some guiding design principles would be quite nice, especially with regards to big things like #489.