DatabaseManager: do not insert movies one by one

[SebastienTouze]

Hi,

In DatabaseManager (and MainWindows where the calls are made), the movies in a directory are added one by one.
The use of QSqlQuery::prepare once, then QSqlQuery::bindValue() should be more efficient for multiple movies additions.

[ochurlaud]

Do it if you know how to handle it.

[ochurlaud]

To do this: Create a QList of QStringList in the addNewMovie() function. Then send one big request when the we all movies have been detected