Ensure dropins folder works

JMC has a dropins folder in the root directory of the JMC app,
ensure this actually works, which it does not seem to at them moment.

The idea is that the user can place a plugin directly in the folder, and the plugin is loaded when JMC launches.
One usecase for this is for users to be able to share their exported custom made JFR pages.

The dropins folder is supposed to work like this: https://help.eclipse.org/oxygen/index.jsp?topic=%2Forg.eclipse.platform.doc.isv%2Freference%2Fmisc%2Fp2_dropins_format.html
but reading in different forums it's functionality is a bit shaky.
It might be a good idea to figure out if the dropins folder worked in JMC 6 or not.