QuickEditor: Use XCB API to position the editor window

Qt scales coordinates relative to the upper-left corner of the screen
where the quick editor window is. This means that (0, 0) in the logical
pixels is not the same as (0, 0) in the native pixels.

Commit ccf8d522 tried to address that
issue by mapping (0, 0) from the native pixels to the logical pixels.

Unfortunately, it doesn't work reliably because setting geometry may
move a window to another screen and thus the computed window position
will be wrong.

It would be wonderful if QXcbWindowFunctions had a function that sets
the window geometry in native pixels that we could use here in Spectacle
to bypass HiDPI code path.

As a short term solution, this change makes Spectacle use XCB API
directly to position itself.