ToolbarButtons with icons and labels not rendered correctly

[mbektas]

Description

If a ToolbarButton is created with label and icon then icon and label are shown as separate rows that are centered, instead of shown as single row (label after the icon) as before.

Reproduce

new ToolbarButton({
      iconClassName: 'jp-AddIcon',
      label: 'Add Timestamp',
     ...
});

It could be caused by changes in #7236

[jasongrout]

CC @telamonian

What versions of js packages and jupyterlab do you have?

[mbektas]

In sync with master. Most ^2.0.0-alpha.0. Tested using mbektas@4171b71

[telamonian]

@jasongrout @mbektasbbg #7266 fixes this. Some extra SVG-specific styling was getting added to icons in the case where icon handling falls back to the old "icons as CSS background images" behavior.

Not coincidentally, the SVG styling for toolbar icon labels is also broken, so I'll fix that too in the same PR