Add z_index parameter for EmptyStateManager

[calderonsamuel]

Have you read the Contributing Guidelines?

Issue #52

Description

• EmptyStateManager has now an additional z_index argument, to set
  the stack level for the empty state container. Fixes [Bug]: Empty state content overlays modal dialog #52
• I added tests for when the user sets this parameter and for when doesn't do it.
• I removed the css rule for .empty-state-container since it is no longer used

Definition of Done

• The change is thoroughly documented.
• The CI passes (R CMD check, linter, unit tests, spelling).
• Any generated files have been updated (e.g. .Rd files with roxygen2::roxygenise())