Ensure minimum delay between subsequent key presses

Our usual way of ensuring that remote control keys are pressed at
the right time is:
```py
stbt.press(key1)
stbt.wait_for_match(template)
stbt.press(key2)
```
Due to the enhanced template match algorithm introduced in commit
ac97d6dd, wait_for_match might return so fast that some set-top-boxes
miss the second key press. According to our measurement results
these set-top-boxes require a certain minimum delay between subsequent
key presses in order to register every key.

This commit adds a new optional parameter `interpress_delay_secs` to
`stbt.press` that specifies a minimum time to wait after the preceding
key press that `stbt.press` ensures. The default value of this parameter
is read from the parameter of the same name in the `[global]` section of
the stbt config file.