Add function to get window visibility

Closes #562

Co-authored-by: devomaa <lmao@distruzione.org>

src/main/resources/data/computercraft/lua/rom/apis/window.lua

@@ -474,6 +474,14 @@ function create(parent, nX, nY, nWidth, nHeight, bStartVisible)
         end
     end
 
+    --- Get whether this window is visible. Invisible windows will not be
+    -- drawn to the screen until they are made visible again.
+    --
+    -- @treturn boolean Whether this window is visible.
+    -- @see Window:setVisible
+    function window.isVisible()
+        return bVisible
+    end
     --- Draw this window. This does nothing if the window is not visible.
     --
     -- @see Window:setVisible

src/test/resources/test-rom/spec/apis/window_spec.lua

@@ -157,4 +157,17 @@ describe("The window library", function()
             expect({ w.getLine(1) }):same { "test ", "aaaa0", "4444f" }
         end)
     end)
+    describe("Window.setVisible", function()
+        it("validates arguments", function()
+            local w = mk()
+            expect.error(w.setVisible, nil):eq("bad argument #1 (expected boolean, got nil)")
+        end)
+    end)
+    describe("Window.isVisible", function()
+         it("gets window visibility", function()
+           local w = mk()
+           w.setVisible(false)
+           expect(w.isVisible()):same(false)
+         end)
+    end)
 end)