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>