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.
This caused by the setting singleton instance (actually,
QStandardPath::writableLocation) was used before QApplication
was constructed, resulting it doesn't know the application
name. So we manually append the name to the path...
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.
