Utilities/Sphinx: Allow explicit target for genex

Split the genex directive into its own class, allowing a slight
simplification of CMakeObject. Add ability to specify an explicit target
name for the same.

Use this to provide a target for the `$<TARGET_PROPERTY:prop>` generator
expression which is otherwise missing one (due to overlap with
`$<TARGET_PROPERTY:tgt,prop>`).  With this one can write:

    :genex:`$<TARGET_PROPERTY:prop> <TARGET_PROPERTY:prop>`

to link the second variant.

Fixes: #24573