X11: Use x11-dl instead of x11

[prokopyl]

This PR replaces the x11 crate with its x11-dl counterpart, which allows to dynamically link against the various X11 libraries we depend on.
(Namely: libX11, libGLX, libX11-xcb, libxcb, and probably a few more)

This brings us more in line with the rest of the Rust GUI ecosystem (e.g. winit, glutin, wry all rely on x11-dl rather than x11), as well as a rather big part of the audio plugins ecosystem (most notably, JUCE dynamically loads all Xlib symbols instead of statically linking against libX11).

For us, the biggest improvement is that it allows plugins that use baseview to still function on linux systems that lack one or more X libraries (only GUI creation will fail, which AFAIK most hosts handle gracefully enough).

This PR also fixes an use-after-free bug which could occur if the GLContext outlived its Window (it was using a raw display pointer, now replaced with an Rc to the XcbConnection).
This and related changes also removes quite a few uses of unsafe throughout the library.