add changelog dialog and display_file api (implemented in osx and ios)

data/layout.xml

@@ -583,7 +583,7 @@
     </border>
   </layout>
 
-    <!--About window-->
+  <!--About window-->
   <layout id="about">
     <border positioning="absolute" position="0 0" color=".4 .4 .4 .8" width="100%" height="100%" align="center" justify="center">
       <border thickness="1" border-color=".2" pad="3" max-width="650">
@@ -644,6 +644,46 @@ Roboto Font License:
     </border>
   </layout>
 
+  <!--Changelog window-->
+  <layout id="changelog">
+    <border positioning="absolute" position="0 0" color=".4 .4 .4 .8" width="100%" height="100%" align="center" justify="center">
+      <border thickness="1" border-color=".2" pad="3" max-width="650">
+        <border width="100%" height="30" color=".2 .2 .2 .9" dir="row" align="center" justify="center">
+          <text text="What's new in PanoPainter" font-face="arial" font-size="11"></text>
+        </border>
+        <border width="100%" color="0 0 0 .9" pad="10" dir="col">
+          <!--window content-->
+          <border color=".2" width="100%" height="300">
+            <scroll pad="10" margin="5" color=".3 .3 .3 .4" dir="row" wrap="1" shrink="1">
+              <text margin="0 0 0 5" font-face="arial" font-size="30" text="0.1.1"/>
+              <text margin="10 0 0 5" font-face="arial" font-size="11" text=
+"This is the first public release of PanoPainter. It's important to note this is an early
+access to the software, so be aware it may crash or behave in unexpected ways.
+Always save your work. It comes with a lot of features that will be improved over time.
+Here's a list of what's available in this release.
+- Immersive 360° panoramic canvas
+- High-Performance GPU Accelerated Paint Engine
+- Intuitive gestures to navigate
+- Dynamic cross-platform User Interface
+- Native Apple Pencil support
+- Native Sonar Pen support
+- Massive resolution up to 8K panorama
+- Highly customizable brushes
+- Cloud Storage for device interoperability
+- Timelapse
+- Layers
+"/>
+            </scroll>
+          </border>
+          <!--footer buttons-->
+          <node height="40" grow="1" dir="row" align="flex-end" justify="flex-end">
+            <button id="btn-ok" text="Ok" width="100" height="30" margin="0 10 0 0"/>
+          </node>
+        </border>
+      </border>
+    </border>
+  </layout>
+  
   <!--popup menu-->
   <layout id="popup-menu">
     <popup-menu positioning="absolute" position="100 100" width="150" thickness="1" border-color=".1" color=".4 .4 .4 .8" dir="col">