jsroot: fix several problems with tabs layout

- when creating new tab, add it to direct child of top element,
otherwise new tab can appear in other nested tabs
- tab actiavted before drawing otherwise it is not visible and
has wrong dimension,
- SVG method GetBBox is not working for hidden elements in firefox,
make workaround to prevent JavaScript exception

Signed-off-by: Bertrand Bellenot <bertrand.bellenot@cern.ch>