Fix elevation prompt on Windows

[AdrasteonDev]

This pull request is related to #42.

I've added elevation at the start of a command execution (when elevation is required), to rerun the command with administrator rights on Windows (except in debugging, to allow the use of breakpoints).

As AddonsInstallCommand only needs to be elevated if an addon has a "symlink" source in the addons file, I've added elevation only if at least one addon has a "symlink" source.

For information, I've added two methods (FileContains in FileClient and AddonsFileContainsSymlinkAddons in AddonsFileRepo) without adding tests for them for the moment. I'll try to add them this week-end.

Let me know if you have any questions/remarks. I've checked the option to allow edits, so feel free to add commits directly to the forked repo if needed.

[jolexxa]

Wow, thanks! :D Since you've done the hard part, I'd like to take a pass at this and see if I can move some stuff around to make this a little bit simpler and easier to test.

[jolexxa]

I can't push a new branch to your fork, and I don't want to just throw commits on top of what you're doing because I need your help reviewing and testing on Windows. I've gone ahead and merged this and opened up a refactor at #51 — mind helping me with it?

[paulloz]

You can now remove UACHelper from the dependencies too 😉

[AdrasteonDev]

I can't push a new branch to your fork, and I don't want to just throw commits on top of what you're doing because I need your help reviewing and testing on Windows. I've gone ahead and merged this and opened up a refactor at #51 — mind helping me with it?

I'll take a look at it :)