Add FileUtil.makeExecutable(Path)

[hohwille]

In several PRs we have already discussed about making a file executable (see resolved review comments):

• #505: create startup script command #540 (Startscript IntelliJ)
• #25: implement tool commandlet for intellij #297 (IntelliJ)
• #13: implement tool commandlet for aws cli #122 (Aws)

To avoid copy&paste and other hacks, we should create a clean solution:

• Add new method makeExecutable(Path) to FileAccess
• Implement that in FileAccessImpl so that it behaves like chmod a+x on the given file.
• If the given Path does not exist, the method should fail.
• On Windows it should behave like chmod a+x «file» in git-bash (do nothing) but on all other OS it should add the executable flags (without modifying other file attributes).