Add assertions and suppress deprecation warnings in theme-manager-spec

The new expected behavior is now tested. The deprecated behavior is
also tested until we remove it later.
Remove deprecated Project::setPath call
Fix spec name
:lipstick: grammar
Separate IPC messages that don’t belong in the command palette

Path opening and update signaling were both using the command-sending
IPC mechanism, but neither is actually a command. This commit adds a
second “message” channel with custom handling on the render process
side for these messages, rather than attempting to route them through
commands.