show bottom sheets / dialogs / menus based on screen size

Fixes #76

lib/helpers/extensions/is_desktop.dart

@@ -0,0 +1,7 @@
+import "package:material_ui/material_ui.dart";
+
+extension IsDesktop on BuildContext {
+  /// Whether the window is wide enough for the desktop layout, with the
+  /// sidebar always visible, menus instead of bottom sheets, etc.
+  bool get isDesktop => MediaQuery.sizeOf(this).width > 650;
+}