Fixes #539: Allow windows to be set always_on_top

[twitchyliquid64]

Fixes #539.

[xxx@xxx]:~/projects/iced> cargo test
    Finished test [unoptimized + debuginfo] target(s) in 0.10s
     Running target/debug/deps/iced-cbc05f1843ac8fee

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

   Doc-tests iced

running 7 tests
test src/lib.rs -  (line 69) ... ok
test src/sandbox.rs - sandbox::Sandbox (line 65) ... ok
test src/application.rs - application::Application (line 58) ... ok
test src/widget.rs - widget (line 7) ... ok
test src/lib.rs -  (line 53) ... ok
test src/lib.rs -  (line 127) ... ok
test src/lib.rs -  (line 80) ... ok

test result: ok. 7 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

[hecrj]

Great! Thank you!