Add cancel button to add-plugin dialog

Closes #343

resources/resources.qrc

@@ -6,6 +6,7 @@
         <file>16x16/application-exit.png</file>
         <file>16x16/arrow-right.png</file>
         <file>16x16/configure.png</file>
+        <file>16x16/dialog-cancel.png</file>
         <file>16x16/dialog-error.png</file>
         <file>16x16/dialog-information.png</file>
         <file>16x16/dialog-ok-apply.png</file>

resources/ui/carla_database.ui

@@ -339,6 +339,17 @@
        </property>
       </widget>
      </item>
+     <item>
+      <widget class="QPushButton" name="b_cancel">
+       <property name="text">
+        <string>Cancel</string>
+       </property>
+       <property name="icon">
+        <iconset resource="../resources.qrc">
+         <normaloff>:/16x16/dialog-cancel.png</normaloff>:/16x16/dialog-cancel.png</iconset>
+       </property>
+      </widget>
+     </item>
     </layout>
    </item>
   </layout>

source/carla_database.py

@@ -1162,6 +1162,7 @@ class PluginDatabaseW(QDialog):
 
         self.finished.connect(self.slot_saveSettings)
         self.ui.b_add.clicked.connect(self.slot_addPlugin)
+        self.ui.b_cancel.clicked.connect(self.reject)
         self.ui.b_refresh.clicked.connect(self.slot_refreshPlugins)
         self.ui.tb_filters.clicked.connect(self.slot_maybeShowFilters)
         self.ui.lineEdit.textChanged.connect(self.slot_checkFilters)