Finish preliminary offline support

* ProgressProvider now has an abort() call
* Abort button support added to the progress dialog
* YggdrasilTask and MojangAccount adapted to support abort

YggdrasilTask will time out after 10 seconds of no network activity, or when the user pushes the Play Offline button.
In offline mode, all instance update tasks are skipped! This will need further work.

gui/dialogs/ProgressDialog.ui

@@ -7,7 +7,7 @@
     <x>0</x>
     <y>0</y>
     <width>400</width>
-    <height>68</height>
+    <height>100</height>
    </rect>
   </property>
   <property name="minimumSize">
@@ -25,8 +25,8 @@
   <property name="windowTitle">
    <string>Please wait...</string>
   </property>
-  <layout class="QVBoxLayout" name="verticalLayout">
-   <item>
+  <layout class="QGridLayout" name="gridLayout">
+   <item row="0" column="0">
     <widget class="QLabel" name="statusLabel">
      <property name="text">
       <string>Task Status...</string>
@@ -36,7 +36,7 @@
      </property>
     </widget>
    </item>
-   <item>
+   <item row="1" column="0">
     <widget class="QProgressBar" name="taskProgressBar">
      <property name="value">
       <number>24</number>
@@ -46,6 +46,19 @@
      </property>
     </widget>
    </item>
+   <item row="2" column="0">
+    <widget class="QPushButton" name="skipButton">
+     <property name="sizePolicy">
+      <sizepolicy hsizetype="Expanding" vsizetype="Expanding">
+       <horstretch>0</horstretch>
+       <verstretch>0</verstretch>
+      </sizepolicy>
+     </property>
+     <property name="text">
+      <string>Skip</string>
+     </property>
+    </widget>
+   </item>
   </layout>
  </widget>
  <resources/>