[Linux] - remove destination file or directory before copying to it.

The current implementation in SDLCore::copyDiskItem can have unexpected behaviour when dealing with directories. I don't know how to concisely explain it...

'cp -R src/Dir dst/Dir' will make dst/Dir a copy of src/Dir (as probably intended); however if dst/Dir exists it will copy src/Dir into the directory dst/Dir which results in dst/Dir/Dir being a copy of src/Dir (probably not intended). This can be seen in the IDE by publishing to the same directory twice in a row and will leave you with something like 'exportPath/Linux/Linux'.

On many distros you can change the command to 'cp -RT src/Dir dst/Dir' and even if dst/Dir exists it will result in dst/Dir being a copy of src/Dir. However, I don't think the '-T ' switch is a Linux standard and have no clue how to find out.

Instead I use removeDiskItem on the destItemPath before doing the cp. This means copyDiskItem("src/file", "Dir") will now remove Dir and replace it with a copy of src/file. To copy src/file into Dir you would need to do copyDiskItem("src/file", "Dir/file"). I suspect this is expected/intended behaviour of copyDiskItem across Win and Mac as well; if not, a different fix is necessary.