set up interaction register push/pops for windows & views; use to implement top-level cursor-breakpoint operations; more convergence with new unified src view path

src/df/gfx/df_gfx.h

@@ -199,6 +199,8 @@ struct DF_View
   
   // rjf: view state
   UI_ScrollPt2 scroll_pos;
+  TxtPt cursor;
+  TxtPt mark;
   
   // rjf: ctrl context overrides
   DF_CtrlCtx ctrl_ctx_overrides;
@@ -836,6 +838,8 @@ read_only global DF_View df_g_nil_view =
   0,
   {0},
   {0},
+  {0},
+  {0},
   0,
   0,
   0,