Allow undefined actions in keys configuration file

In the keys file there are three possibilities for each action:

1. One or several keys are assigned to it
2. It is marked as UNDEFINED (new)
3. It is missing from the file

On load of the keys file, calcurse respectively

1. Assigns the key(s)
2. Assigns "UNDEFINED" (new)
3. Assigns a default key if possible

If default keys were assigned, the user is informed of the number of
actions affected, and the keys file is updated.

After load each action must either have keys assigned or be undefined.
If not, calcurse exits with a failure. If there are syntax/semantic
errors in the file, calcurse rejects the file and exits.

When an interactive user leaves the keys configuration menu, a warning
is issued if any action is UNDEFINED. The keys file is always updated.

Addresses GitHub issue #298.

Additionally: Description of concepts and data structures used for
keyboard keys and virtual keys (actions) as well as name changes and
comments to improve readability.

Signed-off-by: Lars Henriksen <LarsHenriksen@get2net.dk>
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/help.c

@@ -105,7 +105,7 @@ int display_help(const char *topic)
 
 	if (!io_file_exists(path)) {
 		int ch = keys_str2int(topic);
-		enum key action = keys_get_action(ch);
+		enum vkey action = keys_get_action(ch);
 		if (ch > 0 && action > 0 && action != KEY_UNDEF) {
 			topic = keys_get_label(action);
 			mem_free(path);