QtIFW

Update documentation for Component Selection Page

Repository categories introduced in IFW 3.1 can cause behavior changes
in older automation scripts that use component selection methods.
Update documentation to include new items on Component Selection Page
and provide an example code for fetching components from a category.

Task-number: QTIFW-1354
Change-Id: Icf93d1325563d3bdf6a38799ab260b13f4078a81
Reviewed-by: Tarja Sundqvist <tarja.sundqvist@qt.io>
Reviewed-by: Katja Marttila <katja.marttila@qt.io>