Move popups away from Dialog, introducing new semantic window positioning

This merge requests introduces two new classes.

PlasmaPopupWindow: This is a Plasma Themed window positioned relatively
to another item in another window. Processing the move is batched and
happens in sync with the rendering.

TransientPlacementHint: The API for this class maps closely to the
wayland xdg_positioner. We specify the position semantically and using
code similar to kwin's position the window obeying relevant constraints.

For now on both platforms this then maps to a global co-ordarinte that
we move the window to, longer term the wayland side can use the
positioner directly.

These will both be re-used for Applet popups.

There are some minor behavioural differences introduced in this MR:
 - Popups are resized to the tooltip's implicitSize
 - ToolTipDialog loses all (hopefully) irrelevant properties from Dialog
- ToolTipDialog changes from having a "location" property to a
"popupDirection"

TooltipArea is untouched, so...