add Dialog.fixed function

it allows for displaying dialogs that are not centered

Default method of centering the dialog relies on modifying element `margin-left` - it makes it hard to position the dialog using CSS (for example: to display without the margin on the narrow page etc.) 
This change lets people to declare the dialog as `fixed` in which case we stay away from element style and let application CSS to take over.

It also makes merging component/dialog#13 less disruptive (as things stand today one can undo style modifications, if we do it in async way though all bets are off)