- change default settings to make the default behavior more close to legacy Windows Photo Viewer.
- remove redundant fallback for fetching enum settings.
fix not thread-safe way for fetching settings singleton.
remove qApp from Settings' parent, use nullptr replace it and leak it anyway.
because we may initialize it before initializing QApplication.
in that time, qApp is nullptr.
- remove auto long image mode.
- there is no "auto" requirement for this.
- user can utilize fit by height/width to achieve the same goal by manually click them.
- some optimization code about graphic view are preserved.
fix the minor memory leak issue, also bring some possibility to
implement custom keybinding.
And oops I also did another feature in this commit..
Now we are able to set mouse wheel behavior in config dialog.
