Input box: inherit font from workbench and do not set it

[bpasero]

It looks like the input box is setting the font that is already defined in the workbench: https://github.com/Microsoft/vscode/blob/master/src/vs/base/browser/ui/inputbox/inputBox.css#L21

We should clean this up and have only one place where the font is defined (workbench) so that we a) do not have to repeat all the rules for other languages and b) make it easier to provide an option to configure the font used in VS Code.

[bpasero]

@alexandrudima @jrieken not sure who to assign, I think this would have an impact on the find input and the rename box. I don't see other uses of input box in the editor afaik.