overlay: allow to draw both/either of filled/outlined rectangle

overlay: allow to draw both/either of filled/outlined rectangle

Filled/outlined rectangles shown as snapping overlay are now enabled/disabled
independently with `snapping.overlay.[region|edge].bg.enabled` and
`snapping.overlay.[region|edge].border.enabled`.

To keep the default behavior, `*.bg.enabled` is yes and `*.border.enabled` is
no for hardware-based renderers, while `*.bg.enabled` is no and
`*.border.enabled` is yes for software-based (pixman) renderer.

Users can now use a filled rectangle as an overlay even with pixman renderer.
However, this may severely impact performance when used with translucent
`snapping.overlay.[region|edge].bg.color`.

This commit includes a refactor to use substruct `theme_snapping_overlay`
inside `theme` in order to pass it to `create_overlay_rect()` in a cleaner way.

Breaking changes is:
- `snapping.overlay.[region|edge].fill` is now removed.