wallpapers/image: fix random order update logic
In SlideFilterModel, an added/deleted wallpaper is not always the last
one, so first and last also needs to be taken into account.
m_randomOrder stores the row numbers of the source model in a specific order.
When a new wallpaper is added, the first argument is not always equal
to sourceModel->rowCount(), and when the condition is not met, all the
existing row numbers in m_randomOrder that >= first need to be
increased by 1 (or the number of new wallpapers), to make space for new
wallpapers. Vice versa when wallpapers are removed.
Edited by Fushan Wen