Bootstrap update for v4

[gbjbaanb]

The bootstrap initialisation code (which is a really neat idea BTW) looks for a

temporary element for styling of the main frame,

The panel class was removed in bootstrap 3. In Boostrap 4, we have the "card" class that does the same things.

So the bootstrap.js init code just needs panel replacing with card.

Another niggle with the bootstrap code is that the z-index should be set to make the picker appear above a modal. Bootstrap currently sets the z-index of its modals to be 1050.

[nazar-pc]

I was not using bootstrap integration for ages, so if you're willing to make a PR to make it work with Bootstrap v4 that would be great!

[gbjbaanb]

@nazar-pc consider it done.