frames: Keep information about the ongoing grab operation, and retry if needed.

When a passive touch grab is rejected over the frame, management is punted to
the frame itself, and pointer events emulated, but the attempt to transfer the
grab from the GDK connection to the Clutter one fails with AlreadyGrabbed, and
will fail until the Clutter connection receives the XI_TouchEnd resulting from
XIRejectTouch, gotten after the XI_ButtonPress on the GDK connection.

In order to bypass this shortcoming, store the current grab operation on the
frame as long as the button is pressed, so it is retried once on the next
motion event happening during frame dragging, that will have a recent enough
timestamp to succeed. If no grabbing succeeded, the current grab operation
data will be reset on GDK_BUTTON_RELEASE.