Fix bugs #6871 and #8119.

Fix bugs #6871 and #8119.

Both bugs above were due to a missing screen update. This patch
updates the current view after dispatching a lyxserver command
and thus solves both.

The patch is quite strightforward and the only difficulty was due
to the fact that the lyxserver needs the result of the dispatched
command. Now, GuiApplication::dispatch(FuncRequest const &) does
right this, but does not return the result, while
GuiApplication::dispatch(FuncRequest const &, DispatchResult &)
is called by the former but does not return the result. So, I
split the first one, isolating the code performing the update,
such that the second one can also update the current view when
the caller is the lyx server. When the action is initiated by
anything different from the lyx server, the behavior is unchanged.