add custom ssd height

[cramt]

add option for pop-os/cosmic-epoch#275

[mmstick]

There is already a header size config here: https://github.com/pop-os/libcosmic/blob/master/src/config/mod.rs#L98-L99

[cramt]

There is already a header size config here: master/src/config/mod.rs#L98-L99

yes but that doesnt solve the issue i linked of having no SSD

it could also be a boolean of "ssd_enabled" or something but i initially just went with the more customizable option

[git-f0x]

ssd_enabled should be a compositor config option, since it can just not use the header bar if false.
The compositor sets SSD height on its own, and reducing SSD height would visually break window control buttons (it could only be reduced to 32 without visual bugs), so changes likely aren't needed here?