Hide plugins annotated as invisible

The `org.scijava.plugin.Plugin` annotation type marks ImageJ plugins for
the dynamic discovery mechanism and specifies the plugin name, menu path,
and other metadata. The `visible` element specifies whether the plugin
should be displayed in the user interface. By default, `visible = true`,
and the plugin's menu entry is displayed. However, when `visible =
false`, the plugin's menu entry is not displayed.

This behavior was followed by the modern UI, but ignored by the legacy UI.
By filtering out plugins with the `visible = false` annotation element
from the list of modern plugins passed to the legacy UI, the modern
behavior is preserved.
Merge pull request #47 from bnanes/hidden

Hide plugins annotated as invisible