Add small checkboxes

[KellanClark]

I recently ran into a use case where I wanted a checkbox, but the vertical size needed messed with the space between lines. I looked around the demo and found ImGui::SmallButton() covered a similar case, so I reused its code to make a smaller checkbox.

My usecase in a very WIP debugger window showing what a SmallCheckbox() looks like compared to a Checkbox().

In the middle of text as shown in the demo window.

I understand if the need for this is too rare to justify a new widget and PR. Feel free to reject it.

[ocornut]

Hello Kellan,

Thanks for the PR.
Although this is no doubt useful, presently I would prefer to avoid adding those variants as they tend to easily multiply.

Note that the API design using a namespace means you can add this function (with same ImGui::SmallCheckbox() name) in your own source file so presently given the low complexity of that function I'd recommend people to do that.

One small tangential thing is that I've been considering adding X/Y variant of style variables, e.g. ImGuiStyleVar_FramePaddingY so people inclined to use public Push/Pop API can perform an Y only override more easily.
(Practically speaking, I do prefer the approach of backuping old value on the stack as this tends to be simpler and faster when done in a helper function like this).

Good luck on the emulators :)