Fix tagbar status line

This actually contains two bug fixes.  My check for Powerline was busted.  The plugin doesn't load early enough, so we can't look for the variable as proof-positive.  Instead, we need to check for the absence of the variable.  Next, I noticed that when the tagbar window popped up, the status bar didn't change until you navigated away from the window and back again.  It turns out that autocmds are not being run.  To fix this, I call the `Pl#UpdateStatusline()` after toggling the tagbar.