[Feature]Allow opening incompatible Xcode versions

[ipavlidakis]

This revision, implements the mechanism that allows opening Xcode versions that are incompatible with the current macOS version. That applies on all Xcode versions that have been released with older or later macOS versions than the running one.

With this revision, by tapping Open, we check if the running major version is equal to the selected Xcode's required major version:

• if they are equal, we perform the NSWorkspace openApplication like usual
• if they are not, we construct a URL to the Xcode's binary (/Contents/MacOS/Xcode). Then we instantiate a new process and pass the binary path for execution (we use a Process rather than asking the NSWorkspace to open the binary, to avoid showing the terminal window).