slick-greeter: Add blur effect and fix bugs (#268)

* Convert some icons in SVG format

Improve rendering quality on HiDPI displays.

* Add blur effect and fix bugs

- Add blur effect to the panel(menubar) and dashboard (dashbox).

- Fix a bug that prevented the user list from automatically repositioning in the grid when the session list was closed.

- Fix toggleBox misalignment

- Fix the offset and clipping issues affecting the 'Select Desktop Session' label, caused by the avatar implementation in the grid.

- Remove SMALL_AVATAR_OPACITY to avoid visibility issues.

Author: Hilyxx