Refactor plugin-shared code into a separate header

[ariccio]

Git has a fantastic feature called a subtree that uses nested repositories to manage project dependencies.

Before anything, There's lots of good information about it:

• "Understanding Git Subtree"
• "Using Git subtrees for repository separation"
• "Extracting Parts of Git Repository and Keeping the History"
• "Using Git subtrees to split a repository"

Refactoring some of the headers into a separate repo would make it far easier for plugin developers to build & maintain up to date plugins, as they could update their copy of the headers with a simple git shell command.

I'm not a plugin developer, but as best I can tell, the very minimal headers that a plugin dev needs to include are roughly:

• Notepad_plus_msgs.h
• PluginInterface.h
• Scintilla.h
• menuCmdID.h
• Window.h
• Docking.h (maybe)
• DockingDlgInterface.h (maybe)

I came up with this while studying/fixing multiple buffer overruns in HexEdit, which seems to have last updated its copy of the Notepad++ headers in _~2009_.

[milipili]

A real N++ SDK would be indeed a good thing to have

[milipili]

@donho : could you give me the list of the n++ headers currently strictly required by any plugin ?

[donho]

@milipili
https://github.com/npp-plugins/plugintemplate/blob/master/src/Notepad_plus_msgs.h

[donho]

I think we can close this issue since there's only one file is mandatory.
Of course @milipili please reopen it if you judge it's necessary to reopen it.