Added setting to toggle drawing of title bar separator

Summary:
Adds a new settings on Breeze theme which allows the user to turn off the title bar separator drawn

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Rationale behind this patch:

From what I understand, the separator is seen as a design choice so I won't argue over it, not to mention something like this is entirely subjective.

However, I personally dislike the separator, especially on darker windows.  The only way to get rid of it is to either make a custom color scheme for each application so the colors match exactly or run a patched version of breeze (which is what I do). Before using my Breeze patched version I searched on the net on how to remove it, which led me to several threads with other users wanting to toggle it off which is why I make this patch public.

Below are screenshots where the separator is especially noticeable and in my non-design-expert opinion kind of distracting. I think it's a positive addition. Looking forward to your opinions on the matter.

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Test Plan:
1. Use default breeze theme
2. Check the enable/disable the new setting

Reviewers: #breeze, #vdg, ngraham, hpereiradacosta

Reviewed By: ngraham

Subscribers: andreaska, abetts, jensreuterberg, broulik, rpelorosso, #breeze, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D8362