Fix sizeHint for IdealToolButton

As title says:
Use sizeFromContents from style rather than fixed spacing and margin, to get
the correct size hint. This is similar to what is done in QToolButton and fixes
truncated text for styles that add extra spacing (consistently with
sizeFromContents) when rendering toolbuttons (this is the case for breeze)

Note: this also removes a "FIXME" comment

REVIEW: 119898
Merge remote-tracking branch 'origin/1.7'