adding preference option for dynamic input blocking

/inpblock is now having subcommands 'timeout' and 'dynamic'
with:

/inpblock timeout <milliseconds>

and

/inpblock dynamic <on|off>

Defaults are:

/inpblock timeout 500
/inpblock dynamic on

To get the old behavior specify:

/inpblock timeout 20
/inpblock dynamic off

The dynamic mode will block incrementally after something
should be written to the window or after a key was pressed. So pressing
a key would set the timeout to 0ms and after 10 timeouts to the next
bigger one.

Example (with dynamic mode on):

"/inpblock timeout 50"

timeout series:

10x 0ms
10x 10ms (0ms + 10 times since last keypress)
10x 30ms (10ms + 20 times since last keypress)
*x50ms until next key was pressed or

src/config/preferences.h

@@ -100,7 +100,8 @@ typedef enum {
     PREF_OTR_WARN,
     PREF_OTR_POLICY,
     PREF_RESOURCE_TITLE,
-    PREF_RESOURCE_MESSAGE
+    PREF_RESOURCE_MESSAGE,
+    PREF_INPBLOCK_DYNAMIC
 } preference_t;
 
 typedef struct prof_alias_t {