mach: Added runtime application options

Reused mach.Options for run time options. It is set with
Engine.setOptions function. ``pub const options`` on top level App has
no effect and will be ignored completely.

Added a blank struct StartupOptions which would be used for startup time
options in future. Currently they aren't used for anything.

src/structs.zig

@@ -12,6 +12,9 @@ pub const SizeOptional = struct {
 };
 
 /// Application options that can be configured at init time.
+pub const StartupOptions = struct {};
+
+/// Application options that can be configured at run time.
 pub const Options = struct {
     /// The title of the window.
     title: [*:0]const u8 = "Mach engine",