Allow change of Settings Directory in the launcher

Currently, you cannot type in the `Settings Directory` section of the launcher, and the `Open` button opens the `.chunky` directory.

This means you cannot change the settings directory location - replacing `Open` with a `...` button similar to the one seen in `Java Runtime` could allow you to browse to the `.chunky` folder.

Also, since `Minecraft directory`, `Settings directory` and `Java Runtime` look like a text boxes similar to `Java options`, it may be a good idea to allow users to enter the file paths manually.