This adds a slider to set the compression quality for lossy file formats
to the settings page, so users can set it themselves instead of having
to just accept the (in my opinion too low) default value.
The slider defaults to a quality level of 90.
A couple of notes:
-When adding the slider to the 'Save'-page layout I noticed that the bottom of the help text for the filename chooser got cut off because the window was too small. I tried to figure out how to get the settings page to scroll but couldn't quite get it working. Instead I resorted to simply increasing the default size of the settings window just enough to make it fit. If someone with more experience in Qt than me can get the page to scroll OR can suggest a better way to display the formatting help text (which I feel takes up too much of the page), that would be marvelous. -When saving an image at full (100) quality it still looks a bit washed out and fuzzy, when compared to, for example, saving the image as png and converting it to JPG in gimp at full quality. This might be an issue with QImageWriter unless this is the desired behaviour? I haven't looked into it too much, just something I've noticed when testing