refactor lua show_dialog

The lua show dialog used to get the video object from resources,
which is not ideal. In this version, lua kernel base internalizes
a pointer to the video object, and show_dialog is bound to a
method of the lua kernel base.

From now on, lua kernels which want to show dialogs need to be
constructed with a pointer to a CVideo, if it is null the lua will
fail with a runtime error.

The application lua kernel is constructed with the video for
game_launcher, and the game lua kernel is modified so that it is
owned by the play controller and not the game events manager, and
held in a boost scoped pointer rather than manually deleted. This
is because the game events manager doesn't have a video object,
also it conceptually seems independent of the lua kernel anyways.

The map generator lua kernels are constructed with NULL, but if a
user configuration dialog is provided (new feature) then when they
try to run it they will update with the mp_create's video pointer.