Extend content model of image for metadata, alt references

[robander]

Based on: https://lists.oasis-open.org/archives/dita/201801/msg00061.html

Discussed at TC Jan 30, 2018; primary concerns are backwards compatibility (reluctant to remove href/scope/format from <image> and force <imageref> usage for the common case), and concern from Robert about how the base topic module can include imagelabel as a specialization of data -- better to make that a new base element to go along with new imageref. Left at stage 1 for now; Eliot already at max count for later stages.