File picker should support multiple selection

[wildasin]

Describe the bug
There appears to be no way to import more than one .zim file at a time into the library.

Expected behavior
When picking files to add to the library, it should be possible to select more than one by long-pressing several file icons in succession. Ideally, it would be possible to drag to select a range of files a la Google Photos, and a Select All would be provided.

Steps to reproduce the behavior:

1. Click '+' in the Library.
2. Long-press a file icon to select it.
3. Long-press a second file icon.
4. The first file becomes deselected in favor of the second; it is not possible to have more than 1 selected.

Environment

• Version of Kiwix Android : 3.13.0
• Device : Galaxy Tab A9+
• OS version : 14

[kelson42]

The idea is good, but at this stage not sure this is worth the complexification of the code.

[wildasin]

It looks like this started to get implemented, because long-pressing a file selects it and it says "1 Selected".

An interim solution that might require less UI development effort might be to long-press the enclosing folder (from one level up in the tree) and be able to add it in one shot.

[wildasin]

When considering how to prioritize this issue, it might help if I described my user journey. I'm not sure if it's common but I don't think it's too odd.

When I found out about Kiwix the other day I decided to load up a 1TB SD card to install on my tablet. I set up the various torrents and web downloads to grab most of the available ZIMs, which took about a day but operated unattended.

Once all these files were on the SD card, I expected to be able to open them up in Kiwix and use them; however, between this issue and #4182 it will be at least another day of fairly intensively babysitting my tablet, when it could easily have been accomplished in seconds.