Use '~' prefix instead of '%' for random brushes

[akien-mga]

Fixes #337.

It's not necessarily the best fix but according to https://en.wikipedia.org/wiki/Filename#Reserved_characters_and_words ~ doesn't seem to be reserved, so it might be a safer bet.
It works for me on Linux but I haven't tried other OSes to confirm.

Note that a ~ suffix is used by some code editors to mark backup versions of changed files, which might be a source of confusion (but does show that ~ in filenames seems to be safe). It doesn't seem to conflict with ~ that expands to $HOME, at least in bash, as long as you don't use ~ alone or with ~/somepath.

[akien-mga]

Note also that this would break compatibility for users who currently rely on the % prefix, who would need to rename their brushes. This could be improved to keep support for both prefixes (should be easy), or as mentioned in #132 (comment), the whole system could be changed to something a bit more advanced.

I'm completely fine with this not being merged as is, I use it for my distro package as a workaround but it's fine with me if another, longer term solution is worked on for #337.

[OverloadedOrama]

Thanks for taking the time to fix this. We will most likely change how the brush system works, although I'm not sure yet how it will be and when it will happen, but for now this PR will do!