Guide installer users where to download more QBSPs

Since Qt 5.12 the number of QBSPs in the online installer has reduced a lot. The targets have been moved from installer to account.qt.io downloads section (http://blog.qt.io/blog/2018/12/10/additional-device-creations-targets-available-qt-account/). The developer experience in the installer (and maintenance tool) is such that people do not understand where to find the downloadables and against reality conclude, that we no longer have support for specific hardwares in latest Qt releases. This is not true.

We need to address the glitch on developer experience and discoverability of the account.qt.io/downloads

Possible solution: To ensure better discoverability and make sure that each developer easily finds the QBSPs we should add the following text into the end of component list in installer Qt DC targets: "More targets available in account.qt.io/downloads as QBSP"