Draw flat, borderless, semi-transparent and sharp-cornered notes (#2827)

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* Draw flat, borderless, semi-transparent and sharp-cornered notes; Increase the contrast of the grid

* Convert the note gradient to a horizontal one

* Increase opacity for better visibility

* Reinstate borders, make the fill semi-transparent

* Some cosmetic touch-ups

* Make border width themeable

* Set a boolean for borderless properties

* Rename borderlessNotes to noteBorders

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