Reduce the timeout in KItemListKeyboardSearchManager to 1 second

It turned out that the longer timeout, introduced recently in
02eab49b, not only made multi-letter
keyboard searches easier, but also had some unwanted side effects. I
hope that 1 second, which is between the previous value of 5 seconds and
the pre-KDE 4.8.2 value of 0.4 seconds, is a compromise which will fit
most users' needs.
We will try to improve the situation further in future releases by
providing visual feedback about the keyboard search, but such a change
would not be suitable for a bug-fix release.
CCBUG:297458
CCBUG:297488