Xcode: Add option to choose the startup target

right now in Xcode projects default target is ALL_BUILD. Our company have many single-target projects and we do switching between branches and projects really often. Right now CMake doesn't provide any support for default Xcode target, but have same function for Visual Studio named VS_STARTUP_PROJECT. I added the same variable for Xcode named XCODE_STARTUP_PROJECT, but when this variable is not set "ALL_BUILD" target is choose by default, as it works right now

Topic-rename: xcode-startup-target