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

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