Annotation editor support

[Earthcomputer]

Implements a few features / hooks to support the annotations editor.

The annotations editor is also ready to try out on my branch of yarn.

The features in question are:

• Implements icons in the gutter, and adds the ability for plugins to add them for declarations.
• API for plugins to get the active GUI scale.
• API for plugins to check whether the GUI is set to a dark theme.
• API for plugins to create a JEditorPane the same as the main one in Enigma.
• API for plugins to get the declaration under the cursor (as opposed to only the reference).
• API for plugins to list all classes including library classes (the existing API only listed project classes).
• API for plugins to create a LocalVariableEntry, so that they can get the yarn names of parameters.
• Exposed BridgeMethodIndex to the API.

These features are for the most part in individual commits, but there are some of the commits are fixes to features added in earlier commits.