For workspace containers, swayc_change_layout also changes ->layout alongside
->workspace_layout when it's a sensible thing to do. There is an additional test
for 'layout toggle' command which ensures that containers will be tiled
horizontally after toggling from tabbed or stacked.
Add swayc_change_layout function, which changes either layout or
workspace_layout, depending on the container type. Workspace being
always L_HORIZ makes this much more i3-compatible.
