File Browser: Display vital signs while preparing to display files

[sy2002]

This enhancement idea is inspired by a Discord conversation that starts here:
https://discord.com/channels/719326990221574164/794775503818588200/1232420616154517647

It is about a folder with "a lot of files" in it. First the user claims/thinks that the core/file-browser is frozen, but then "after a while" it displays this warning:

While it is in general advisable to split folders with a ton of files into sub-folders (as people in Discord advise in the above-mentioned conversation), we should not confuse users by a seemingly "frozen" state.

Instead - and this is what this enhancement idea is all about: We should show a progress bar, or if not possible due to unknown overall amount, some "movement" - or as a minimum after a few seconds some "Please wait... Parsing might take up to 30 seconds" or something like that.

[sy2002]

P.S. This will need an M2M enhancement, too and therefore other cores will also benefit from this.