Add alternative rightclick behavior overlay to gui/design

docs/fixrightclick.rst

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+fixrightclick
+================
+
+.. dfhack-tool::
+    :summary: Adjust properties of caravans on the map.
+    :tags: fort interface bugfix
+
+This overlay changes the behavior of the right mouse button and other keys mapped to "Leave screen" to only reset the selection rectangle when painting designations, constructions, minecart tracks, zones, etc., instead of outright quitting the painting mode. It can be toggled in the UI Overlays tab of `gui/control-panel`.
+
+Usage
+-----
+
+::
+
+    overlay enable|disable fixrightclick.selection

fixrightclick.lua

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+--@ module = true
+
+local overlay = require('plugins.overlay')
+
+RightClickWidget = defclass(RightClickWidget, overlay.OverlayWidget)
+RightClickWidget.ATTRS{
+    desc='When painting a rectangle, makes right click cancel selection instead of exiting.',
+    default_enabled=true,
+    viewscreens={
+        'dwarfmode/Building/Placement',
+        'dwarfmode/Designate',
+        'dwarfmode/Stockpile/Paint',
+        'dwarfmode/Zone/Paint',
+        'dwarfmode/Burrow/Paint'
+        },
+}
+
+local selection_rect = df.global.selection_rect
+local buildreq = df.global.buildreq
+
+function RightClickWidget:onInput(keys)
+    if keys._MOUSE_R or keys.LEAVESCREEN then
+        -- building mode, do not run if buildingplan.planner is enabled since it already provides this functionality
+        if buildreq.selection_pos.x >= 0 and not overlay.get_state().config['buildingplan.planner'].enabled then
+            buildreq.selection_pos:clear()
+            return true
+        -- all other modes
+        elseif selection_rect.start_x >= 0 then
+            selection_rect.start_x = -30000
+            selection_rect.start_y = -30000
+            selection_rect.start_z = -30000
+            return true
+        end
+    end
+end
+
+OVERLAY_WIDGETS = {selection=RightClickWidget}