Fix too small result list when favorites are added

Alexander Lohnaurequested to merge
work/fix_kicker_result_height into Plasma/5.19

We can use the height of both the favoriteSystemActions favoriteApps sidebars to fill out all the space, BUG 422398.

Now it looks like:

Screenshot_20200612_201834

Instead of:

Screenshot_20200612_201952

@ngraham @abetts

Edited by Alexander Lohnau

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