no selection if State didn't receive mouse_press

This change needs to get backported to all the other forks. We want to
retain the ability to draw other UI elements in our apps, and that means
we can't always assume that a mouse press and mouse release will happen
together. Instead, each layer should manage its own state machine, and
only check for events from the immediately lower layer.

This idea generalizes commit c3cdbb52a.

edit.lua

@@ -159,6 +159,7 @@ end
 
 function edit.mouse_press(State, x,y, mouse_button)
   if State.search_term then return end
+  State.mouse_down = mouse_button
 --?   print_and_log(('edit.mouse_press: cursor at %d,%d'):format(State.cursor1.line, State.cursor1.pos))
   if y < State.top then
     State.old_cursor1 = State.cursor1
@@ -209,6 +210,7 @@ end
 
 function edit.mouse_release(State, x,y, mouse_button)
   if State.search_term then return end
+  State.mouse_down = nil
 --?   print_and_log(('edit.mouse_release(%d,%d): cursor at %d,%d'):format(x,y, State.cursor1.line, State.cursor1.pos))
   if y < State.top then
     State.cursor1 = {line=State.screen_top1.line, pos=State.screen_top1.pos}

select.lua

@@ -11,7 +11,7 @@ function Text.clip_selection(State, line_index, apos, bpos)
   -- min,max = sorted(State.selection1,State.cursor1)
   local minl,minp = State.selection1.line,State.selection1.pos
   local maxl,maxp
-  if App.mouse_down(1) then
+  if State.mouse_down then
     maxl,maxp = Text.mouse_pos(State)
   else
     maxl,maxp = State.cursor1.line,State.cursor1.pos