Toolbutton: Fix layout issues

- Improve spacing between icon and text
- Do not enforce a minimum width

Having a minimum width does not make much sense for a ToolButton:
1. It should aim at using the minimum amount of horizontal space when
used in a ToolBar.
2. It looks unbalanced when used with an icon because the content is
flushed to the left, leaving a large amount of white-space on the right.

REVIEW: 104735