Fixing reference to Dialog's backdrop element so it closes properly

[sbosell]

Relates to #379

There should only be one backdrop element, so the service now will hold a reference to this backdrop element so that it can be properly removed.

[pkozlowski-opensource]

@sbosell thnx for this PR but the current $dialog code is kind of broken from the design point of view and I'm in process of rewriting it right now in scope of #441

Prefer to focus on rewrites instead of patching broken code. Thnx for your work and heads up though!

[i8ramin]

Not sure if this is related, but somehow I was getting negative values for activeBackdrops.value when navigating through my app. Adding this bit of logic around line 224 inside the _removeElementsFromDom method fixed the problem for me. Looking forward to the rewrite.

if (activeBackdrops.value > 0) {
  activeBackdrops.value--;
}