Selection insert fix

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Fixes #80, #41 https://forum.nette.org/cs/24391-standartni-html-formular-a-zpracovani-post-pozadavku-v-presenteru-jine-stranky#p163128

In Structure there is autoincrement information in columns which i need in insert method but i'm not sure if i can extend it interface so i add method for this only into Selection.

Any ideas how to get this information in Selection better way?

I also changed return value when inserting multiple values. So insert can returns ActiveRow or number of affected rows.

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@dg i've updated this, so what do you think? or #105 way should be better?