alternative style for named menu section headers

This introduces an optional new/modern look for named menu section
headers to the Qt4 and Qt5 style plugins. The new design uses left-
aligned text, underlined and rendered in bold, with an overlying
separator that is also left-aligned. Alignment is done w.r.t. the
check/icon position, and of course flips when switching to a right-to-
left rendering mode.

https://phabricator.kde.org/D5286