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.