Improve ZoomComboBox

Eugene Popovrequested to merge
epopov/gwenview:zoomcombobox into master

This MR slightly improves the ZoomComboBox:

  • improve changing value by scrolling and pressing the up/pageup/down/pagedown keys
  • improve line edit behavior whilte entering a value
  • improve converting value to text and vice versa

Merge request reports