add ZScreenModal class for modal dialogs

docs/changelog.txt

@@ -55,6 +55,7 @@ changelog.txt uses a syntax similar to RST, with a few special sequences:
 
 ## Lua
 - ``widgets.RangeSlider``: new mouse-controlled two-headed slider widget
+- ``gui.ZScreenModal``: ZScreen subclass for modal dialogs
 
 ## Removed
 - `title-version`: replaced by an `overlay` widget

docs/dev/Lua API.rst

@@ -4323,6 +4323,13 @@ Here is an example skeleton for a ZScreen tool window::
 
     view = view and view:raise() or MyScreen{}:show()
 
+ZScreenModal class
+------------------
+
+A ZScreen convenience subclass that sets the attributes to something
+appropriate for modal dialogs. The game is force paused, and no input is passed
+through to the underlying viewscreens.
+
 FramedScreen class
 ------------------
 

library/lua/gui.lua

@@ -867,8 +867,17 @@ function ZScreen:onGetSelectedPlant()
     return zscreen_get_any(self, 'Plant')
 end
 
---------------------------
--- Framed screen object --
+-- convenience subclass for modal dialogs
+ZScreenModal = defclass(ZScreenModal, ZScreen)
+ZScreenModal.ATTRS{
+    defocusable = false,
+    force_pause = true,
+    pass_pause = false,
+    pass_movement_keys = false,
+    pass_mouse_clicks = false,
+}
+
+-- Framed screen object
 --------------------------
 
 -- Plain grey-colored frame.