fix(tabs): fix tab content compiling wrong (Closes #599, #631, #574)

* Before, tab content was being transcluded before the tab content area
 was ready. This forced us to disconnect the tab contents from the DOM
 temporarily, then reocnnect them later. This caused a lot of problems.
* Now, neither the tab content or header are transcluded until both
 the heading and content areas are loaded. This is simpler and fixes
 many weird compilation bugs.