Add encodeFilePath to FileUtils to convert file paths into encoded strings (replace metacharacters such as %, @, $, #, etc.)

Cannot use encodeURI/encodeURIComponent because they replace `#` with `%25`, which is incorrect - should be replaced by `%23`.

I'm not sure if there is a Brackets' utility for this already (I searched for it but couldn't find it).

I've tested with `test#tt.jpg` and `test%tt.jpg`, but nothing else - are there any other special characters that should be added to this replace scheme?

Should this be moved to a module in `utils/` and loaded from ImageViewer?

Fixes #9117.
And btw, we usually use something like `expect(...).toEqual(xy);` or `.toBe(xy)`.