bugfix: fix bundled plugins on MacOS (#1540)

[midwan]

The bundled plugins of capsimg and floppybridge, were not included in the final app bundle. Furthermore, they need to be signed with the same digital certificate, otherwise MacOS will block them from loading.

Fixes #1540 .

Changes proposed in this pull request:

• include bundled plugins in a separate folder
• digitally sign bundled plugins during the build step
• during the first run, if the plugins directory did not exist, copy the bundled files in there as well.

@midwan

[solskogen]

What happens with the plugins in the bundle are newer than the ones in the plugin directory?

[midwan]

@solskogen the way macOS works, no plugins will be allowed to be loaded, unless they were signed with the same certificate the app bundle was. In other words, you can't drop in your own plugins in there. :)

Regarding existing files: only if the directory was missing, will the copy take place. If you already had a Plugins directory in your Amiberry folder, it won't be affected.

[solskogen]

By default the directory doesn't exist, which means that when a user upgrades Amiberry the newer plugins won't be copied.

[midwan]

If the directory is missing, it will be created and the plugins copied in there.
If it exists, it will be left untouched.