Allow sampling devices to be refreshed while SDRangel is running.

(Possibly need to add a mutex for MainCore::getPluginManager, so only
can be used by one thread)
Add option to automatically update My Position based on GPS. This is
started in MainCore, so we get position as soon as possible.
Don't set QFileDialog::DontUseNativeDialog on Android, as Qt's file
dialog can't access user storage. Set globally for Linux/Windows, rather
than for each dialog.

 MainWindow:
 - Add welcome dialog for Android.
 - Don't show menu bar or status bar on Android to save screen space.
 - On Android, change tab position when screen orientation changes.
 - Load default configurations and presets first time SDRangel is run.
 - Change loadConfiguration to use a QProgressDialog rather than
   QMessageBox, as the latter was crashing on Android.
   - Use DialogPositioner to ensure dialogs are on screen.

sdrbase/plugin/plugininterface.h

@@ -66,6 +66,7 @@ public:
 		int deviceNbItems;        //!< Number of items (or streams) in the device. >1 for composite devices.
 		int deviceItemIndex;      //!< For composite devices this is the Rx or Tx stream index. -1 if not initialized
 		int claimed;              //!< This is the device set index if claimed else -1
+        bool removed;             //!< Set if device has been removed
 
 		SamplingDevice(const QString& _displayedName,
                 const QString& _hardwareId,
@@ -85,8 +86,17 @@ public:
 			streamType(_streamType),
 			deviceNbItems(_deviceNbItems),
 			deviceItemIndex(_deviceItemIndex),
-			claimed(-1)
+			claimed(-1),
+            removed(false)
 		{ }
+
+        bool operator==(const SamplingDevice& rhs) const
+        {
+            return displayedName == rhs.displayedName
+                && hardwareId == rhs.hardwareId
+                && id == rhs.id
+                && serial == rhs.serial;
+        }
 	};
 	typedef QList<SamplingDevice> SamplingDevices;