Several minor fixes to xpybutil

* Updated `keybind` to pass the event object to callbacks like `mousebind` does.
* Fixes for several functions: `set_wm_pid`, `set_wm_pid_checked`, `request_desktop_geometry`, `request_desktop_geometry_checked`, and `get_keysym`
* Documentation corrections around the reason for using `request_*` functions instead of `set_*`.
* Fixes to event routing for `MapRequestEvent` and `SelectionRequest`.

Sorry for the large lump of commits; I had forgotten to do pull requests for these, so they piled up a bit.