Custom picker: Make preview and selection touches more like in gallery apps

[nicolas-raoul]

in the custom picker user taps to select the picture and long presses to open it.

In gallery apps, one taps to preview, and long-presses to select.

The current implementation is because multi-presses selection is hard to implement.

By "multi-presses selection" I mean long-press one or several pictures, then leaving some pictures unselected, then continue selecting later pictures, with the ability to drag in order to select many pictures without having to tap each of them individually:

https://github.com/user-attachments/assets/cf55d0bd-a0e4-42a2-b092-0eae5955b7ab

Sub-tasks of this task, all to implement in the custom picker:

1. Make tap the action to preview a picture.
2. Make long-press the action to select a picture.
3. Implement multi-presses selections.

A pull request must implement 1&2&3, not only 1&2. That's because with only 1&2 the app would become less efficient to use. Thanks! :-)

[rohit9625]

I would like to work on it 😀