Menu item and/or keyboard shortcut to open current image in external editor

[IonoclastBrigham]

When browsing through collections, especially recently-imported or -downloaded images, it is common to find ones that may require cropping, resizing, or color adjustment. Currently, because Ivy doesn't support any image editing features (apart from XMP tags), the best way to do this is to copy the image path and open it in an editor program.

This issue proposes to add a new UI/UX element to open the image on-demand in the user's preferred editor.

Default Editors and Cross-platform Support

Each target platform could have a prioritized list of one or more likely image editor programs, as the default. For instance, on macOS, every user will have Preview.app installed, which can scale, rotate, crop, and color-correct images in a lightweight manner.

Other platforms, such as GNU/Linux, may have differing software installed depending on the distro and user configuration. In that case, the most common editors can be checked for, preferring lightweight image browser type applications first, leading up to larger tools and suites like the Gimp or Krita.

User Preferences

The default editor, in turn, is only meant to be that: a default. The user should be allowed to specify their favorite image editing tool with one or more of an exported environment variable, an entry in the Ivy config file (with UI support for setting that), and possibly an ephemeral session-setting which is forgotten when Ivy exits.

This proposal takes no position on which mechanisms should be implemented for setting an editor override, other than that the ordering for precedence should be well-defined, and generally follow the Principle of Least Surprise™.

Dependencies

Doesn't rely on, but would be greatly enhanced by #105, which would take care of reloading the image automatically after being edited.