Preserve entity visibility while switching active group.

Switching active group by itself is not an editing but a viewing
action; the active group is not recorded in the savefile. However,
the entity visibility status is, and this is annoying when source
control is used, because e.g. looking up dimensions in one of
the inner groups whose display was turned off ends up changing
the savefile.

When the display has to be turned on manually, this modification
of the file becomes explicit, so there's no longer any question
of what action modified the file.

This can also be convenient when inserting a group in the middle
of the stack, which will be implemented in the future.

src/textscreens.cpp

@@ -69,8 +69,6 @@ void TextWindow::ScreenShowGroupsSpecial(int link, uint32_t v) {
 }
 void TextWindow::ScreenActivateGroup(int link, uint32_t v) {
     hGroup hg = { v };
-    Group *g = SK.GetGroup(hg);
-    g->visible = true;
     SS.GW.activeGroup.v = v;
     SK.GetGroup(SS.GW.activeGroup)->Activate();
     SS.GW.ClearSuper();