[DataGrid] Implement internationalization

[DanailH]

One iteration on #196

Overview

I'm this is the first part of the "Implement Localization" ticket. For the second part there is going to be a follow up ticket. This feature is available on both the DataGrid and XGrid components.

What does this PR solves?

- We add a localeText prop to the DataGrid and XGrid components. This is an object that can be globally overridden through the theme with theme.components.MuiDataGrid.defaultProps.localeText.
- The prop performs a deep merge with the default translations.
- The default translations are in the en_US locale.
- We use the context to propagate the prop down to all the components of the grid. This propagation can be implemented with a custom context or the apiRef, to be determined on what we need to optimize. No matter the solution we pick, it needs to propagate everywhere in the data grid. I memo shouldn't prevent the value to be updated.
- We document it.

I've added a useI18n hook that has only one exported method for now that is the getText. It is used to get the text value that is assigned to a key in the 'localText' object.

Later on we can extend the hook with additional functionality needed for part 2.

Documentation, tests and full text replacement is missing from this Draft. Once we have the implementation and the naming 😅 nailed down I'll add in the missing things and convert the Draft into a normal PR.

Example usage:

<DataGrid localeText={{ densityText: 'size' }} />

ToDo:

• Add one smoke test
• Update docs https://material-ui.com/components/data-grid/localization/
• Update API pages https://material-ui.com/api/data-grid/, https://material-ui.com/api/x-grid/
• Use getText to display the text in the grid (on one component initially, a follow up PR will be created for the rest of the components)

[oliviertassinari]

Great idea to only translate a few keys!

[DanailH]

I've fixed the outstanding issues and comments, added a test case and updated the docs.

Note: I've only converted the text in the Density Selector component for now to make the PR more focused on the functionality. Once this is merged I'll open a follow up PR that handles the rest of the text used in the grid.

updated