Begin child with size param

[techmindful]

Not ready to merge yet, but issue can be discussed.

This is either a pull request or an issue. The current bindings to functions like ImGui::BeginChild doesn't cover optional params. When working on my app, I needed to set a child window's size, which is such an optional param. I tried to modify beginChild in place to take more params, but that broke some example build targets, since they didn't supply enough params to the new beginChild.

So I think the options are:

1. Modify functions like beginChild in place, and fix the code for example build targets.
2. Create a new function named like beginChildOfSize or beginChildWithOptParams, which is what this branch is currently doing.

[ocharles]

See also #33

[techmindful]

See also #33

From that pull request, one option is to write something like beginChildFull, similar to what I'm doing.
Another option seems to be at the bottom, with generic-lens. I know the basics of lens, but haven't dived deep enough to understand any of the lens usage occurred there. Are you familiar with lens enough to further decisions on that?

[dpwiz]

Upon closer inspection it is the other way around - beginChild should use all the arguments, and "default arguments" mode of operation is a "trick".

I've got this sorted in #93