The Image's source property used to be a string. Now it is a Resource
enum, which can either be None or an absolute file path to the image on
disk. This also replaces the internal Image type.
The compiler internally resolves the img bang expression to a resource
reference, which shall remain just an absolute path. For now the target
generator passes that through, but in the future the target generator
may choose a target specific way of embedding the data and thus
generating a different Resource type in the final code (through
compile_expression in the cpp and rust generator).
The C++ binding is a bit messy as cbindgen doesn't really support
exporting enums that can be constructed on the C++ side. So instead we
use cbindgen to merely export the type internally and only use the tag
from it then. The public API is then a custom Resource type that is
meant to be binary compatible.
Remove the Optional from the evaluation context passing for property
evaluation. Unfortunately there are nullptr uses left on the C++ side,
that need to be replaced with passing through.
The LoweredItem and LoweredComponent contained, in essence, the same
information as the Element and Component in object_tree. Since the
moving declarations pass moved everything to the root element and the
LoweredPropertyDeclarations have been removed as well, this is the last
step.
This type that allowed indexing the property declarations in the hierarchy
to the properties moved to the root is not needed anymore.
For now a simple set remains in the LoweredItem to generate
correct accessor prefixes.
