command: export some option metadata

This might be interesting for GUIs and such.

It's probably still a little bit insufficient. For example, the filter
and audio/video output lists are not available through this.

options/m_option.c

@@ -696,7 +696,7 @@ static char *print_choice(const m_option_t *opt, const void *val)
 }
 
 const struct m_option_type m_option_type_choice = {
-    .name  = "String",  // same as arbitrary strings in option list for now
+    .name  = "Choice",
     .size  = sizeof(int),
     .parse = parse_choice,
     .print = print_choice,

options/m_property.h

@@ -189,6 +189,8 @@ struct m_sub_property {
     .type = {.type = CONF_TYPE_STRING}, .value = {.string = (char *)(s)}
 #define SUB_PROP_FLOAT(f) \
     .type = {.type = CONF_TYPE_FLOAT}, .value = {.float_ = (f)}
+#define SUB_PROP_DOUBLE(f) \
+    .type = {.type = CONF_TYPE_DOUBLE}, .value = {.double_ = (f)}
 #define SUB_PROP_FLAG(f) \
     .type = {.type = CONF_TYPE_FLAG}, .value = {.flag = (f)}