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UV/Image Editor: Overlay Popover

The overlay options in the image/uv editor is hidden in side panels and menus. Sometimes this panel is even hidden, while still useful.
The goal of this task is to introduce an overlay pop-over just like the overlay-popover of the 3d viewport.

Popover has
* UV Stretching (only available in the UV mode, when active object mode is a mesh and in OB_EDIT mode)
* Display As (only available in the UV mode, when active object mode is a mesh and in OB_EDIT mode)
* Show Modified (only available in the UV mode, when active object mode is a mesh and in OB_EDIT mode)
* Show UV Edges (including opacity slider; available UV, View, Paint, when active object mode is a mesh and in OB_EDIT mode)
* Udim tiles when no image is available.

Like the 3d viewport, there will be a editor toggle to enable/disable the overlays

For compatibility reasons the RNA properties are added to both the `SpaceImage.uv_editor` amd `SpaceImage.overlay`. On DNA level they are still stored in the SpaceImage. only new properties are added to the SpaceImageOverlay struct. During the next major release we could remove these options from `SpaceImage.uv_editor`. This should be noted in the Python section of release notes.

Reviewed By: Julian Eisel, Pablo Vazquez

Differential Revision: https://developer.blender.org/D8890