Show a warning if the instance path contains a '!'

Checks the instance path whether it contains '!' and if yes warns the user. This is done because java has problems handling paths with a '!' correctly, so Minecraft chrashes when Forge tries to load mods. There were already two issuses with this problem, but I can't find them now. :(

The checks and warnings happen the time MMC loads (via QLOG_WARN <- Mistyped that in the commit message), the time the GUI starts (via a dialog) and when the user changes the instance path via the settings window (only if there's a problem).

Format should be correct, but I'm not sure as clang-format doesn't seem to work on my Windows (it completely reformats the whole code). If there are still errors (or stuff that I overlooked) or something else, feel free to ask me to fix it (I'm pretty new to C++ programming, especially Qt).
Also use QString::contains instead of manually iterating through the characters.