VKEYBD: Add small keyboard pack for virtual keyboard.

This pack contains just the 320x240 layout.

This is a quick way to achieve the result of a smaller keyboard
when running on 640x480 and larger games as requested bu a user.

To use this, the pack zip file, vkeybd_small.zip should be placed
in the extrapath in the same way as vkeybd_default.zip.

However, it is also necessary to set this as the active virtual
keyboard layout by adding the following linein the global [scummvm]
section of the configuration file i.e. scummvm.ini:
vkeybd_pack_name=vkeybd_small

This should be added to the GUI as a misc option, in future.

Currently, the closest layout size to the overlay size is chosen
automatically by the code in the method:
bool VirtualKeyboardParser::parserCallback_mode(ParserNode *node)

This should probably also be made configurable in future to allow
smaller keyboard sizes to be chosen.