Long press on fullscreen image to add to favorites

**Is your feature request related to a problem? Please describe.**
It is difficult to add images to favorites on mobile. The only way to do it currently is to use the dropdown menu in the folder viewer. The favorite option doesn't even exist in the dropdown menu when an image is in fullscreen!

**Describe the solution you'd like**
Long hold the image to favorite (perhaps haptic feedback and a star appearing at the middle of the screen to confirm it has been favorited)

**Describe alternatives you've considered**
Add favorites to the fullscreen image dropdown menu at the very least.

**Additional context**
It would be nice to be able to simply scroll through albums in fullscreen by swiping left/right and long hold on images that you want favorited. Having to back out of the menu and then use the folder's dropdown menu for the file to favorite said file is very slow and cumbersome when you want to sort through hundreds if not thousands of pictures.