Added the Credits dialog

src/windows/ui/credits.ui

+<?xml version="1.0" encoding="UTF-8"?>
+<ui version="4.0">
+ <class>Dialog</class>
+ <widget class="QDialog" name="Dialog">
+  <property name="geometry">
+   <rect>
+    <x>0</x>
+    <y>0</y>
+    <width>400</width>
+    <height>300</height>
+   </rect>
+  </property>
+  <property name="windowTitle">
+   <string>Dialog</string>
+  </property>
+  <property name="windowIcon">
+   <iconset>
+    <normaloff>../../../xdg/openshot.svg</normaloff>../../../xdg/openshot.svg</iconset>
+  </property>
+  <widget class="QTabWidget" name="tabWidget">
+   <property name="geometry">
+    <rect>
+     <x>10</x>
+     <y>10</y>
+     <width>380</width>
+     <height>231</height>
+    </rect>
+   </property>
+   <property name="locale">
+    <locale language="English" country="UnitedStates"/>
+   </property>
+   <property name="currentIndex">
+    <number>3</number>
+   </property>
+   <widget class="QWidget" name="tab">
+    <attribute name="title">
+     <string>Written by</string>
+    </attribute>
+    <widget class="QListView" name="listViewwritten">
+     <property name="geometry">
+      <rect>
+       <x>0</x>
+       <y>0</y>
+       <width>381</width>
+       <height>201</height>
+      </rect>
+     </property>
+    </widget>
+   </widget>
+   <widget class="QWidget" name="tab_2">
+    <attribute name="title">
+     <string>Documented by</string>
+    </attribute>
+    <widget class="QListView" name="listViewdocumented">
+     <property name="geometry">
+      <rect>
+       <x>0</x>
+       <y>0</y>
+       <width>381</width>
+       <height>201</height>
+      </rect>
+     </property>
+    </widget>
+   </widget>
+   <widget class="QWidget" name="tab_4">
+    <attribute name="title">
+     <string>Translated by</string>
+    </attribute>
+    <widget class="QListView" name="listViewtranslated">
+     <property name="geometry">
+      <rect>
+       <x>0</x>
+       <y>0</y>
+       <width>381</width>
+       <height>201</height>
+      </rect>
+     </property>
+    </widget>
+   </widget>
+   <widget class="QWidget" name="tab_3">
+    <attribute name="title">
+     <string>Artwork by</string>
+    </attribute>
+    <widget class="QListView" name="listViewartwork">
+     <property name="geometry">
+      <rect>
+       <x>0</x>
+       <y>0</y>
+       <width>381</width>
+       <height>201</height>
+      </rect>
+     </property>
+    </widget>
+   </widget>
+  </widget>
+  <widget class="QPushButton" name="btnclose">
+   <property name="geometry">
+    <rect>
+     <x>290</x>
+     <y>260</y>
+     <width>87</width>
+     <height>27</height>
+    </rect>
+   </property>
+   <property name="text">
+    <string>Close</string>
+   </property>
+  </widget>
+ </widget>
+ <resources/>
+ <connections>
+  <connection>
+   <sender>btnclose</sender>
+   <signal>clicked()</signal>
+   <receiver>Dialog</receiver>
+   <slot>reject()</slot>
+   <hints>
+    <hint type="sourcelabel">
+     <x>299</x>
+     <y>268</y>
+    </hint>
+    <hint type="destinationlabel">
+     <x>183</x>
+     <y>268</y>
+    </hint>
+   </hints>
+  </connection>
+ </connections>
+</ui>