使用info.plist设置启动图，不用asset和launch storyboard
图片用如下格式命名，尺寸如info.plist中所示，拖到项目中：
图片下载：https://download.csdn.net/download/slionls/11235407
<key>UILaunchImages~ipad</key> <array> <dict> <key>UILaunchImageSize</key> <string>{1024, 1366}</string> <key>UILaunchImageOrientation</key> <string>Landscape</string> <key>UILaunchImageName</key> <string>Default-iOS12-Landscape-1024h</string> <key>UILaunchImageMinimumOSVersion</key> <string>12.0</string> </dict> <dict> <key>UILaunchImageSize</key> <string>{834, 1194}</string> <key>UILaunchImageOrientation</key> <string>Landscape</string> <key>UILaunchImageName</key> <string>Default-iOS12-Landscape-834h</string> <key>UILaunchImageMinimumOSVersion</key> <string>12.0</string> </dict> <dict> <key>UILaunchImageSize</key> <string>{834, 1112}</string> <key>UILaunchImageOrientation</key> <string>Landscape</string> <key>UILaunchImageName</key> <string>Default-iOS11-Landscape-834h</string> <key>UILaunchImageMinimumOSVersion</key> <string>11.0</string> </dict> <dict> <key>UILaunchImageMinimumOSVersion</key> <string>7.0</string> <key>UILaunchImageName</key> <string>Default-iOS7-Landscape</string> <key>UILaunchImageOrientation</key> <string>Landscape</string> <key>UILaunchImageSize</key> <string>{768, 1024}</string> </dict> </array>
