Size and positions of forms

I make some changes on how the size of the forms is calculated the first time it's opened, delegating it to AbstractNavtable. Take a look to this pull request on navtable (https://github.com/navtable/navtable/pull/69)

Some bugs about the persistance of the size and position of forms have been fixed. Most os this code can be extrated to a new class to avoid pollute AbstractForm, but i think that is not a blocker. Let me now if you don't think the same.