Editor GUI uses its own SelectSavegameDialog instance

An instance of a window is bound to a specific window manager. The old
code used the main_gui's dialog, which means that the ingame window
manager never knew about this new window.

As a side effect, the editor ingame gui doesn't need a reference to the
main gui anymore. Yay.
Remove dependency on main gui in game's IngameGui

Similar to the previous commit, by using main_gui's show_popup the
ingame window manager doesn't know about this new window.
However, every window manager knows how to display popups, so we just
use our own for that.
Drop main_gui parameter from ingame guis

Less coupling, yeah!