overflow-menu as view-more-symbolic, and horizontal

Summary:
use the 3 dots icons as view-more-symbolic, as traditionally in
the iconography of most systems those are for menus of actions
that didn't fit in a toolbar (as the overflow-menu name tells)
while the 3 lines one means the main menu of an application and
should always appear at most in a single place in a whole app
this makes it less confusing as users will use our apps together
apps written in other toolkits, web pages where this is common as
well, as well apps on their phones which will all use this paradigm.

adapt the horizontal version too, to look like what firefox uses
tough i would make it black as well, as now it may always look disabled

Test Plan:
shown the mixer applet (which will need its own rr) which looks lighter
and causes less confusion between the global panel menu icon and the little
overflow menus of the mixer items

Reviewers: VDG, andreaska

Reviewed By: andreaska

Subscribers: Fuchs, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D11137