qt: merge walletstack and walletframe

There were too many levels of indirection here, and the functionality of
walletframe and walletstack can easily be merged. This commit
merges the two which cuts a lot of lines of boilerplate code.
qt: remove awkward way of setting GUI pages

Selecting the button for a pages was going through bitcoingui->walletframe->walletview->bitcoingui.
Because of this, the actions for the pages had to be exposed on the BitcoinGUI object.
Merge pull request #3109 from laanwj/2013_10_remove_walletstack

qt: merge walletstack and walletframe