Worked around some quirky Linux file system issues. Namely

the Chromebooks share their file system to Linux using the 9p file
system, which does not support setting file ownership. Newly PrusaSlicer
will detect platform and it will not panick if copy_file() cannot set
file ownership after copying. It just logs the incident, and on
chromebooks the loglevel for that incident is "Info", not "Error".

Adjusted the full screen mode to contain menu bar.
Moved Platform.cpp/hpp to libslic3r

src/libslic3r/Platform.hpp

@@ -0,0 +1,41 @@
+#ifndef SLIC3R_Platform_HPP
+#define SLIC3R_Platform_HPP
+
+namespace Slic3r {
+
+enum class Platform
+{
+	Uninitialized,
+	Unknown,
+	Windows,
+	OSX,
+	Linux,
+	BSDUnix,
+};
+
+enum class PlatformFlavor
+{
+	Uninitialized,
+	Unknown,
+	// For Windows and OSX, until we need to be more specific.
+	Generic,
+	// For Platform::Linux
+	GenericLinux,
+	LinuxOnChromium,
+	// Microsoft's Windows on Linux (Linux kernel simulated on NTFS kernel)
+	WSL,
+	// Microsoft's Windows on Linux, version 2 (virtual machine)
+	WSL2,
+	// For Platform::BSDUnix
+	OpenBSD,
+};
+
+// To be called on program start-up.
+void 			detect_platform();
+
+Platform 		platform();
+PlatformFlavor 	platform_flavor();
+
+} // namespace Slic3r
+
+#endif // SLIC3R_Platform_HPP