As suggested by Joshua, this commit adds the 2-clause BSD license as a
comment block to the top of every source file.
For the first pass, I've just added myself for simplicity. I encourage
everyone to add themselves as copyright holders of any file they've
added or modified in some significant way. If I've added myself in
error somewhere, feel free to replace it with the appropriate copyright
holder instead.
Going forward, all new source files should include a license header.
When a GAction is activated by a menu, or by a toolbar button, you can
now use GAction::activator() to get a pointer to whomever activated it.
This can be used to implement context-specific behaviors in situations
where the same action is exposed through multiple paths.
This addresses an issue that was brought up in #826.
