Refactor logic to determine if stacked/tabbed con

Add a new function `swayc_is_tabbed_or_stacked()` to determine whether or not
the given container has layout of stacked or tabbed. This is done commonly
enough that it should have its own method to help readability and code quality.

No logic changes.

include/container.h

@@ -141,6 +141,11 @@ enum visibility_mask {
 	VISIBLE = true
 } visible;
 
+/**
+ * Whether this con is tabbed or stacked
+ */
+bool swayc_is_tabbed_or_stacked(swayc_t *container);
+
 /**
  * Allocates a new output container.
  */