Fix a bunch of memory leaks in KisPopupPalette (+cleanup)

Summary:

• Reduce duplicate initializations (initializer lists are always

preferable).

• Initialize a few variables that were not initialized before

(m_hovered_Preset, ...)

Allocate QRect on the stack, heap allocations are expensive and were
mostly leaked.

Reviewers: Krita, scottpetrovic

Subscribers: rempt

Differential Revision: https://phabricator.kde.org/D3266