qml: do not unload view in MainViewLoader when model is loading or empty

The requirement of this change has been discussed in the following threads:

• !5890 (comment 477841)
• !5890 (comment 478741)
• !5890 (comment 478790)
• !5890 (comment 478757)
• !5890 (comment 479340)
• !5890 (comment 479425)
• !5890 (comment 479470)