Resolve an error where the brushes from the image/3D viewport
where shared but the tool was not. Causing the brush not to match
the appropriate tool when both an image & 3D viewport were displayed.
Resolve the issue by adding support for a "pending" tool,
a tool ID which is to be used.
This accounts for cases where it's not known if the requested tool
exists and uses the same code paths for initializing tools as is done
for initializing on file load for e.g.
Ref !133085
d42b425ea9