GlobalProps inheritance

[mbohal]

Presently the GlobalProps mechanism does not allow overloading of only specific props. The entire object gets overloaded and thus it is near to impossible to use it to modify GlobalProps only for certain parts of the app.

**Minimal Example: **

<RUIProvider
  globalProps={{
    TextField: { layout: 'horizontal' },
  }}
>
  <RUIProvider
    globalProps={{
      TextField: { size: 'small' },
    }}
  >
    <TextField label="username" />
  </RUIProvider>
</RUIProvider>

Expected GlobalProps: layout=horizontal, size=small
Received GlobalProps: size=small