fix(tab): fix removing of tab panes

Fix for #1125 and parts of #1103. Also fixed keeping track of active tab, so it checks active tab index when adding/removing tabs.

However, like stated in #1103, it's still missing a way of keeping track of tab panes order. The tab directive is assuming tab panes are always added at the end and don't change order. It would be nice to have support for detecting changes in the order of tabs.