[GTK3] Fix menu bugs
Summary: GtkMenus no longer have a curved highlight. Additionally, checkboxes in menus now apply the user's colorscheme properly.
Test Plan:
Menu, before:
Menu, after:
Checkboxes before:
Checkboxes after:
Reviewers: Breeze, ngraham
Reviewed By: Breeze, ngraham
Subscribers: ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D24123