All Unity pop-up dialogues should be centred on the screen
to be consistent with native OS dialogs.

Problem:
Popping windows up in the top right corner gives a feel of Unity's UI has only reached a scaffolding stage - being unfinished.

Suggestions:
Mac behaviour: centred on screen.
Windows behaviour: centred in the main window of the app spawning the dialog, but sometimes centred on screen.

Expected:
Unity should behave as either OS's native dialogs as it gives a more eloquent feel to the UI.