qt: allow user to choose data directory

This adds an introduction screen that is shown when the client is first started in which the user can choose a data directory.

It is also possible to force the intro screen to appear using command line argument `-choosedatadir`.

The data directory is remembered in the QSettings.

The user is warned that the client will download and store 10Gb of data. The intro screen shows how much space is available on the device that contains the chosen directory and warns if this is less than the 10Gb.

To make it possible to translate the introduction dialog, the initialization sequence is changed so that translations are loaded before the data directory. This has the by-effect that it is no longer possible to specify a language in bitcoin.conf inside the data directory.

Please help testing!