Set toolbar/toolbutton font on quickopen line edit

Summary:
The quickopen line edit is shown in the toolbar and thus should
follow the font style of the other tool bar actions. So far,
it did not do that. Now we manually force the toolbar font to
ensure the font style is homogeneous.

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D9481

(cherry picked from commit d0705822)