Add credit for icons used, upscale exchange icon

* Jason Cho gave his express permission to use the icon (it was made 10 years ago and he doesn't have the source files anymore)
* Used waifu2x to upscale the icon
* Used GIMP to draw dark outline around the icon
* Source files are included

qt/dg.qrc

@@ -5,7 +5,7 @@
     <file alias="plus">../images/plus_8.png</file>
     <file alias="minus">../images/minus_8.png</file>
     <file alias="search_clear_13">../qtlib/images/search_clear_13.png</file>
-    <file alias="exchange">../images/exchange_purple.png</file>
+    <file alias="exchange">../images/exchange_purple_upscaled.png</file>
     <file alias="zoom_in">../images/old_zoom_in.png</file>
     <file alias="zoom_out">../images/old_zoom_out.png</file>
     <file alias="zoom_original">../images/old_zoom_original.png</file>

qtlib/about_box.py

@@ -42,7 +42,7 @@ class AboutBox(QDialog):
         self.setWindowTitle(
             tr("About {}").format(QCoreApplication.instance().applicationName())
         )
-        self.resize(400, 190)
+        self.resize(400, 290)
         sizePolicy = QSizePolicy(QSizePolicy.Fixed, QSizePolicy.Fixed)
         sizePolicy.setHorizontalStretch(0)
         sizePolicy.setVerticalStretch(0)
@@ -69,6 +69,21 @@ class AboutBox(QDialog):
         self.verticalLayout.addWidget(self.label_3)
         self.label_3.setText(tr("Licensed under GPLv3"))
         self.label = QLabel(self)
+        self.label_4 = QLabel(self)
+        self.label_4.setWordWrap(True)
+        self.label_4.setTextFormat(Qt.RichText)
+        self.label_4.setOpenExternalLinks(True)
+        self.label_4.setText(tr(
+            """<img src=":/exchange" alt="Exchange" width="16" height="16"> icon
+ made by <a href="http://jasoncho.ca/"> Jason Cho</a> (used with permission).
+<br>
+<img src=":/zoom_in" alt="Zoom In" width="16" height="16">
+<img src=":/zoom_out" alt="Zoom Out" width="16" height="16">
+<img src=":/zoom_best_fit" alt="Zoomt Best Fit" width="16" height="16">
+<img src=":/zoom_original" alt="Zoom Original" width="16" height="16">
+ icons made by <a href="https://findicons.com/pack/1035/human_o2">schollidesign</a>
+ (licensed under GPL)."""))
+        self.verticalLayout.addWidget(self.label_4)
         font = QFont()
         font.setWeight(75)
         font.setBold(True)