[X11] Create windows with a parent window

[t-sin]

This PR can create windows with a user-specified parent window. It's intended to realize what discussed in issue #159 for X11.

EDITED: For compatibility, this introduces a new method new_x11_any_thread_with_root() to create such windows. I considered to change the signature of new_x11_any_thread(), it takes a parent window ID, but I cannot decide that so simply create a new method. I want to discuss about this point.

The way above was an wrong way. For now this PR adds with_x11_parent() method to WindowBuilder by importing winit::platform::unix::WindowBuilderExtUnix, we can set X11 window ID as a parent with it. It seems good because it's similar way in supporting child window in Windows (#96).

By the way this PR is for my VST3 plugin using vst3-sys so I don't have any ideas to test this PR without VST3 codes. I want some advice about testing. This PR adds example code and it works.

• Tested on all platforms changed
• Compilation warnings were addressed
• cargo fmt has been run on this branch
• cargo doc builds successfully
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

[t-sin]

This PR cannot create a child window because it fails arround here. I should consider the xinput2. Please wait.

[t-sin]

Some problem when this PR created are resolved. It's ready for review.