aboutdialog: use just "The Bitcoin developers" as tr()-string

- this ensures our new splash screen and this will share a translatable
  string an remove the need for an additional translation
bitcoin: use static QApplication:: instead of app.
bitcoingui: remove 2 unneeded includes
overviewpage: make some code Qt5 compatible

- that changes will be fine for Qt4 also
Merge pull request #2526 from Diapolo/Qt_misc

Bitcoin-Qt: misc small changes