v0.5: Fix Bugs and Improve Features

Bugs Fixed:

• Fix a bug that MulticastLoopback option performs differently on different platforms;
• Fix a bug that file transfer based on TCP doesn't work on Linux;
• Fix a bug that empty message can be sent in textchat.

Features Improved:

• Use macro definition "QT_DEBUG" for local test in Debug mode.

New Features:

• Disable qDebug/qWarning output in Release version;
• Add shortcut keys, system tray icon, button icons, tool tips and translations;
• Use QtSingleApplication to ensure that only one instance would be running at the same time.

Others:

• Reorganize file structure.