Implement setting HostRequirements on a HostRequirementsWidget

Allow passing a `requirements` to the `HostRequirementsWidget` and automatically setting the `use_custom_button` toggle.
Switch from a `OptionalComboBox` to a `OptionalMultiSelectComboBox` for CPU Architecture and Operating System

Implemented getters and setters on many of the widgets

We aren't using PySide6.Slot decorators because apparently they are
unneeded in pure python now?

Signed-off-by: Alex Hughes <[email protected]>