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.

[pyroraptor07]

Tested this on KDE, Windows, and MacOS with the cosmic app template (did have to update the iced submodule) and so far it seems to work pretty well. The only thing I would add (more because of this bug on MacOS really) is that moving the window using the cosmic header bar should probably be disabled when using SSD.

[pyroraptor07]

Also, on MacOS, the cursor doesn't change for the window controls for some reason, but that probably should be looked at separately.