plugin-base: new 'send-delay' property

The plugins can set this property to provide a custom value for the send delay
used for characters sent to the AT port during probing.

The value given to the property should be a guint64 specifying the delay in
microseconds.

src/mm-plugin-base.h

@@ -131,6 +131,7 @@ typedef struct {
 #define MM_PLUGIN_BASE_ALLOWED_VENDOR_STRINGS  "allowed-vendor-strings"
 #define MM_PLUGIN_BASE_ALLOWED_PRODUCT_STRINGS "allowed-product-strings"
 #define MM_PLUGIN_BASE_CUSTOM_INIT             "custom-init"
+#define MM_PLUGIN_BASE_SEND_DELAY              "send-delay"
 #define MM_PLUGIN_BASE_SORT_LAST               "sort-last"
 
 typedef struct _MMPluginBase MMPluginBase;