Add ability to choose starting position

Add 4 positions files (default, pawnhell, mirror and clockwise)
Read positions until end of file in Board::readPositions
Remove Board::setupBoard()
Add backend::InitialPositions enum class
Take argument in Board::readPositions
Add Board::positionsFilenames static const
Add SelectionWindow and InitialPositionsChoice
Don't initialize board in main in GUI mode
Add actions to Window::menuBar
Use childAt in Window::getTileFromPos
Add Window::resetCurrentGame public slot
resetBoard in Window constructor
Create SelectionWindow in Window constructor (after 1 ms)