Part of #8518Closes#8822
The drop downs were missing, because WidgetFunction had a higher score than them and took precedence. The fix was to adjust scores, so drop down is before both argument name and function. Generally, any widget expecting to "overlap" existing code (and possibly further widgets) should go before WidgetFunction.
# Important Notes
* Extended mockExpressionUpdate to accept a subexpression of a node. Also, I removed it from GraphDb - I want this function to have as little impact on app code as possible.
Related to https://github.com/enso-org/enso/issues/8518
These tests already caught one regression in nav breadcrumbs: https://github.com/enso-org/enso/issues/8756
It also provides API for mocking expression updates for arbitrary nodes on the screen. The implementation is a bit convoluted and includes setting a callback on `window`, but it looks like the only possible solution given our architecture and playwright restrictions.