key-binding: new API, for handling sets of key bindings

Up until now, our Wayland seats have been tracking key bindings. This
makes sense, since the seat’s keymap determines how the key bindings
are resolved.

However, tying bindings to the seat/keymap alone isn’t enough, since
we also depend on the current configuration (i.e. user settings) when
resolving a key binding.

This means configurations that doesn’t match the wayland object’s
configuration, currently don’t resolve key bindings correctly. This
applies to footclients where the user has overridden key bindings on
the command line (e.g. --override key-bindings.foo=bar).

Thus, to correctly resolve key bindings, each set of key bindings must
be tied *both* to a seat/keymap, *and* a configuration.

This patch introduces a key-binding manager, with an API to
add/remove/lookup, and load/unload keymaps from sets of key bindings.

In the API, sets are tied to a seat and terminal instance, since this
makes the most sense (we need to instantiate, or incref a set whenever
a new terminal instance is created). Internally, the set is tied to a
seat and the terminal’s configuration.

Sets are *added* when a new seat is added, and when a new terminal
instance is created. Since there can only be one instance of each
seat, sets are always removed when a seat is removed.

Terminals on the other hand can re-use the same configuration (and
typically do). Thus, sets ref-count the configuration. In other words,
when instantiating a new terminal, we may not have to instantiate a
new set of key bindings, but can often be incref:ed instead.

Whenever the keymap changes on a seat, all key bindings sets
associated with that seat reloads (re-resolves) their key bindings.

Closes #931

terminal.h

@@ -14,10 +14,11 @@
 #include <tllist.h>
 #include <fcft/fcft.h>
 
-#include "config.h"
 #include "composed.h"
+#include "config.h"
 #include "debug.h"
 #include "fdm.h"
+#include "key-binding.h"
 #include "macros.h"
 #include "reaper.h"
 #include "shm.h"