feat: add SSD support via config option

[nanuwo]

This PR adds support for SSD in libcosmic via a config option.
CSD is default on Linux, but will be disabled if the option in the toolkit config file or in app::settings::Settings are true.
On MacOS and Windows, SSD is default: see the last comment on #523. It will be disabled if the option in the toolkit config file or in app::settings::Settings are false.
(I know it's not technically SSD on Mac and Windows but it functions mostly the same)

Fixes:

• [Feature request] Support SSD with a config option #913
• [Suggestion] Maybe have the headerbar fall back to being a menu toolbar when client decorations are disabled? #776
• Close button should be clickable from corner of window when maximised #437 and Make window decorations clickable at the very window edge/corner #487 (when SSD is enabled on a window manager which supports this)
• All COSMIC app windows should have a small neutral border and drop shadows #421 (when SSD is enabled on a window manager which supports this)
• Draw native window controls within the custom titlebar on non linux distributions #523 - partially.