vnc: tight: stop using qdict for palette stuff

vnc: tight: stop using qdict for palette stuff

Profiling with callgrind seems to show that a lot of time is spent
in the palette code (mostly due to memory allocation and qdict to int
conversion).

This patch adds a VncPalette implementation. The palette is stored
in a hash table, like qdict, but which does way less memory allocations,
and doesn't suffer from the QObject overhead.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>