Outline intensity setting

This MR adds a tiny outline intensity combobox to Breeze settings in the Shadow tab. I chose shadow tab since the outline is drawn in the shadow drawcall, so maybe this will help people realise it's not part of the borders.

BUG: 465948 FIXED-IN: 6.0

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Here's examples with default breeze dark theme

Off

Low

Medium

High

Maximum

Currently the effect only appears after pressing OK. This is same with all the Breeze window decoration settings afaik.

Also, dont mind the settings window having same outline at the same time in the images: I'm not using the dev environment so it doesnt affect anything outside of the preview image. In dev env it changes everywhere.