Revert "prepare for channel selector in layer editor"

This reverts commit ae0a0f78f831ff005e9f05be8f10841957dca5b1.
Revert "layerwidget editor now widget-based instead of custom painting code"

This reverts commit 1df0dc2a73122a46cf9f13b5d5787aad92526e6b.
Revert "intermediate step"

This reverts commit 18ddf4218eaf5831a70311a56523051983db6901.