Add tooltips for "Fit" and "Show Thumbnails" actions
"Fit" is a bit non-descriptive so a tooltip is added: "Fit image into the viewing area".
Its text is changed from "Zoom to fit" to "Zoom to Fit" (capital "F") to conform with the HIG.
An expanded tooltip is also added so users which might not understand why smaller images aren't enlarged are aware of the setting "Enlarge smaller images". A hyperlink is added in that help text which opens that settings page.
"Show Thumbnails" also gets an expanded tooltip mostly to make users aware of the cool functionalities and settings it has.
There is also some other useful information in the help texts.
I didn't make pictures or a video. I hope reading the texts from the code is fine.