Thumbnail extraction improvements

[cal2195]

Following on from #527

• The "Add missing thumbnails" button should be placed somewhere more prominent, rather than being hidden behind an unrelated function (Edit folders).

• Running a rescan, either manually hitting "Rescan" or by having continuous watching enabled should also check for missing thumbnails as it finds all the files like the initial scan.

😄

[whyboris]

I think the "add missing thumbnails" as well as "delete old thumbnails" buttons are very ugly which is why I hide them until the user clicks "Edit folders" 😓

I could bring them back to the "Main settings", but thematically they don't fit there -- it feels more coherent for them to be on the "Current Hub" page 😓

I have added a "Welcome" message the first time users open the app which will show exactly this button as the place for all the current-hub screenshot interactions (though this message appears only once ever 😅 )

I think that once all the screenshots are there in the hub it doesn't make sense to re-check all of them every time a user clicks "Rescan". It's not a free operation -- it checks every single file in the /thumbnails, /filmstrips and /clips folders to see if it exists. While it's not that much of a big deal, but it just feels wasteful 🤷‍♂️

"Rescan" may just be adding a single file to the user's "recent" folder -- and it would be great if that operation took seconds, rather than a lot longer with the check of all thumbnails 🤔

I'm tempted to keep everything as is for now, though I too am unhappy about the "add missing thumbs" and "delete old thumbs" buttons 😞 -- I just don't know how to keep the app clean and beautiful without hiding those buttons.

[dia3olik]

What about just checking the filenames and modified dates from an index? It should be pretty fast and it should pickup any recent modification or deletion fast for missing or orphaned thumbs almost automatically.

BTW More importantly I think there should be an option to have these 2 buttons in the main top bar if needed, since with some use cases it's often needed.

Thanks Boris!