display: Establish a separate state variable for routing events

We've long used a switch statement on the grab operation to determine
where events should go. The issue with MetaGrabOp is that it's a mixture
of a few different things, including event routing, state management,
and the behavior to choose during operations.

This leads to poorly defined event routing and hard-to-follow logic,
since it's sometimes unclear what should point where, and our utility
methods for determining grab operations apart can be poorly named.

To fix this, establish the concept of a "event route", which describes
where events should be routed to.
common: Create a better encoding for MetaGrabOp

MetaGrabOp is painful and tedious to work with, because it's a
sequential series of values, meaning we have to use a giant unreadable
switch statement to figure out some basic things about the value.

To solve this, modify the encoding for MetaGrabOp and for the specific
window grab operations so that they're a set of bitflags that we can
easily check.