[Feature request] simple photo collages (combining multiple pictures to one)

[eylenburg]

Checklist

• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I have read the FAQ and my problem isn't listed.
• I have taken the time to fill in all the required details. I understand that the feature request will be dismissed otherwise.
• This issue contains only one feature request.
• I have read and understood the contribution guidelines.

Is your feature request related to a problem? Please describe.
This is just a nice-to-have feature.

Describe the solution you'd like
I would like to have some simple collage support in the editor. Like, having pictures side by side or in a simple grid (2x2, 2 left 1 right, ...). with very basic options for the borders between the pictures (thickness and colour).

Describe alternatives you've considered
Using a separate app like Image Toolbox or MixUp.

Additional context
This feature is included in the Samsung Gallery app and probably some other proprietary gallery apps.

[Uranusek]

There is also PhotoAffix, but it's no longer developed https://github.com/afollestad/photo-affix

Unfortunately, there are not many open source apps for combining photos, so this feature would be very useful.