#10 automatic viewer discovery

[Antonio S. R. Gomes]

scan directory lib/ext/ for viewers and register them
in the viewmanager

Insights:

a) put viewers in jar files

b.1) define a manifest file entry to describe which
classes in the jar must be treated as viewers.

b.2) Another option is just search through all ".class"
files and check if they are concrete implementations of
ontool.app.debugger.Viewer (albeit much more slower on
startup is far more flexible than (b.1))