Fix QFontMetrics::width() deprecation warning by switching to boundingRect().width()

Summary:
According to an article by @cfeck [1], QFontMetrics::width() gave the horizontal advance, which doesn't necessarily include every pixel that the text might use.
He suggested that boundingRect().width() might be what most people actually want.

- [1] https://kdepepo.wordpress.com/2019/08/05/about-deprecation-of-qfontmetricswidth/

Reviewers: #breeze, #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: hpereiradacosta, cfeck, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26270

kstyle/breezestyle.cpp

@@ -2836,7 +2836,7 @@ namespace Breeze
                     if( !menuItemOption->text.isEmpty() )
                     {
                         size.setHeight( qMax( size.height(), textHeight ) );
-                        size.setWidth( qMax( size.width(), menuItemOption->fontMetrics.width( menuItemOption->text ) ) );
+                        size.setWidth( qMax( size.width(), menuItemOption->fontMetrics.boundingRect( menuItemOption->text ).width() ) );
                     }
 
                     return sizeFromContents( CT_ToolButton, &toolButtonOption, size, widget );