Add progress bar to export process

[Palloxin]

Describe your suggested feature

A bar that indicates the export process with a %. I know it is immediate on entries with few chapters BUT i just tried on an entry with 2100 chaps and it takes some time. The problem is idk when it will finish, idk if i can close the app, idk if i can go to other apps without breaking the export process, idk if the process has been broken or i have to just wait more etc... An indicator of the export process is needed.

Other details

I exported an entry with 2172 chaps on an sd card and it took 58 seconds.

Acknowledgements

• [X] I have searched the existing issues and this is a new ticket, NOT a duplicate or related to another open or closed issue.
• [X] I have written a short but informative title.
• [X] If this is an issue with a source, I should be opening an issue in the sources repository.
• [X] I have updated the app to version 1.1.18.
• [X] I will fill out all of the requested information in this form.

[Palloxin]

Since we are already here maybe add it to the importing process too?

[CD-Z]

I have already planed to add this to the export feature as well as some more improvements like the possibility to automatically update the epub if there are new chapters. However for the next half a year I don't have the time, so you have to wait for a bit. Except if someone else wants to do it.

[ovizii]

I'd love to see progress too as I haven't even managed to export anything. I set a folder for the exports and tried exporting, pressing submit and nothing happens, no matter how long I wait, no idea how to debug