[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:
{F7396971}
Menu, after:
{F7396974}

Checkboxes before:
{F7397006}

Checkboxes after:
{F7397009}

Reviewers: #breeze, ngraham

Reviewed By: #breeze, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24123