fmbtvnc: enabler for user-defined mouse button on taps

This patch adds optional "button" parameter to the GUITestConnection
layer when using VNC.
fmbtgti: tap* methods support button parameter

Optional button parameter can be given to tap, tapBitmap, tapItem,
tapOcrText methods. However, if the parameter is given without support
from underlying GUITestConnection layer, an error will raise.