If a window is created on an unfocused workspace, yambar did not
update the empty tag correctly. At least not for persistent
workspaces.
This is because yambar relies on focus events to determine a
workspace's "empty" state. Since the new window, on the new workspace,
isn't focused, there's no focus event, and yambar thinks the workspace
is still empty.
This patch changes the logic slightly; a new workspace is considered
non-empty if it isn't focused (and has a non-zero node count).
Closes#191
b0e132beaf