remove unused whitespaces, add shell.nix and .envrc (#2749)

shell.nix

@@ -0,0 +1,11 @@
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  nativeBuildInputs = with pkgs; [
+    cmake
+    qt5.full
+    qt5.qttools
+    qt5.qtsvg
+  ];
+  buildInputs = [ pkgs.qt5.qtbase ];
+}