Allow plugins to be skipped at runtime

[tresf]

When LMMS starts up on a Linux system without Carla installed, the log shows warnings:

BEFORE:

[carla-hook.sh] Carla does not appear to be installed.  That's OK, please ignore any related library errors.
Cannot load library /home/owner/Downloads/squashfs-root/usr/lib/libcarlabase.so: (libcarla_native-plugin.so: cannot open shared object file: No such file or directory)
Cannot load library /home/owner/Downloads/squashfs-root/usr/lib/libcarlarack.so: (libcarla_native-plugin.so: cannot open shared object file: No such file or directory)
Cannot load library /home/owner/Downloads/squashfs-root/usr/lib/libcarlapatchbay.so: (libcarla_native-plugin.so: cannot open shared object file: No such file or directory)

... however, as can be observed by the logs, we already detected this problem, so we should have a way of informing LMMS to not load plugin at startup.

This PR adds a plugin filter called LMMS_EXCLUDE_PLUGINS that can be set as an environment variable to suppress this by skipping the loading of the a plugin based on name or wildcard (QRegexExpression).

AFTER:

[carla-hook.sh] Carla does not appear to be installed, we'll remove it from the plugin listing.

[messmerd]

I tested it and it worked. I have some minor gripes with code style, and it's not as efficient as it could be, but it only runs once at startup so I'm not really concerned.

[messmerd]

This might fix #5525 or at least work around it

[tresf]

I tested it and it worked. I have some minor gripes with code style, and it's not as efficient as it could be, but it only runs once at startup so I'm not really concerned.

Please provide any feedback that you wish, or if it's easier, please feel free to push directly to this branch.

[tresf]

it's not as efficient as it could be

I know they should be filtered as they're scanned, but I didn't want to remove the ever-so-clean files.unite.