Replace Cliff_Mines_Tiles with decorations that can be added on top of plain cliffs

[wjt]

res://assets/first_party/tiles/Cliff_Mines_Tiles.png is a copy of Cliff_Tiles.png with various mining-related decorations on top:

As @manuq notes on #1613 it would be better for the decorations to be separate tiles that can be placed on top of the plain cliffs:

For 3. I actually wonder if we should treat mines as a decoration layer on top of cliffs. Checking with GIMP I see that they are drawn on top: I have set the cliffs PNG as a layer on top of the cliffs + mines PNG, with "color removal" mode, and the result is a mines-only asset:

Just like we do with this grass decoration: As we add more decorations (bookshelves maybe?) it seems a bit redundant to repeat the cliff texture.

XP Summary (total 225)

Subdomain	Skill	XP
Art: Aesthetics & Visual Design	Problem Solving	75
Art: Aesthetics & Visual Design	Creativity & Design Thinking	75
Art: Aesthetics & Visual Design	Technical Literacy	75

[wjt]

@PlayMatters I think you drew these decorations. I don't suppose you still have the source file around, with the mine decorations on a separate layer?