Clean up code for FlatButtonWithToolTip

- Don't set accessible.name and .description properties, as Qt injects
  these automatically for icons-only buttons set using the display:
  property
- Use the Kirigami duration for how long it takes the tooltip to appear
- Only show a tooltip if the button has remained icons-only and wasn't
  overridden to show text too
- Remove unnecessary imports
- Bump versions of necessary imports