Hiding window decorations

For creating custom popup windows or similar, ability to hide window decorations (also, hiding it from the taskbar) is necessary.

Alternative would be to force everything to use native menus (#193).

For X11, this would involve setting [this](https://specifications.freedesktop.org/wm-spec/wm-spec-latest.html#idm45381393998896) property with one of a variety of options, e.g.:
```java
Atom type = _NET_WM_WINDOW_TYPE_POPUP_MENU;
XChangeProperty(instance->_windowManager.display,
instance->_x11Window,
_NET_WM_WINDOW_TYPE,
XA_ATOM,
32,
PropModeReplace,
(unsigned char *)&type,
1);
```