Installing for MultiMC on macOS not working for self-contained installations.

[OmegaMetor]

Describe the bug
Installer GUI folder picker will not allow you to pick a .app as the folder for a multimc installation on MacOs.

To Reproduce
Steps to reproduce the behavior:

1. Open installer gui.
2. Select MultiMC for install type.
3. Navigate to the location of MultiMC.app
4. You cannot select it.

Expected behavior
I would expect it to be selectable considering a .app is a folder and multimc installations can be self contained inside it's .app

Desktop (please complete the following information):

• OS: MacOS 10.14
• Version: Most recent on website as a JAR.

[LeafHacker]

Not sure how to resolve this, since the file picker is a Java AWT/Swing element and not something we have developed ourselves. Short of building a file picker from scratch or using a file picker from a different library...

You might be able to work around the issue using the installer's command line options.

[elesto]

The other option would be to fix/allow editing the file picker path. Right now if you write the correct path of the multimc folder for mac it doesnt use it when clicking install.