KNSBackend: Do not accept global searches

Ismael Asensiorequested to merge
work/iasensio/kns-global-search into master

Make sure we only show search results from KNewStuff when the user is actively browsing a KNS category (Plasma or Apps add-ons).

This better matches the general user expectation of only reporting app searches by default, when searching from Home and no category is selected.

I think this is an easy low-hanging fruit we can take now with a relative consensus, solving the vast majority of user complains about the Store content in discover. Then other steps or approaches being discussed in #30 can have less user/time-pressure for a better desired result.

BUG: 493047
FIXED-IN: 6.4.0

BEFORE (From "Home") discover-global-search-before-home
AFTER (From "Home") discover-global-search-after-home
AFTER (From "Plasma Add-ons") discover-global-search-after-plasma

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