Introduce KXcbEvent to initialize the memory of sent XCB events

The XCB events were often stack-allocated and partially
uninitialized - valgrind complained in some cases. The XCB
documentation recommends using calloc(32, 1) for them, but we can
also use the stack and initialize the memory using a little
wrapper. Also make sure that the initialized memory has the right
*size*, because according to XCB docs, 32 bytes are always sent,
so 32 bytes should be initialized to prevent leaking information.