x11: comment about gravity

The window "gravity" influences how placement interacts with WM added
borders (i.e. from decorations). This is probably what the code removed
in commit c14721c8 was about.

In theory, we'd probably want to set the gravity depending on the
relative placement requested by the user (so that it's possible to line
up the top/left video pixel with the monitor corner, as well as the
bottom/right pixel - but that would be too complicated, and who cares
after all?).

I'm also not sure whether CenterGravity really uses the top/left corner
as reference point (instead of making coordinates relative to the window
center), but empirically it's correct.
x11: add a wrapper for XGetWindowProperty

XGetWindowProperty is a really bad API, almost as if the NSA designed
it. The wrapper takes care of verifying the return values and handle
corner cases.
x11: fix Drag & Drop

Accidentally broken in commit 95462747.
x11: add wrapper for EWMH XSendEvent calls