add loadMore button in Upload History widget (issue #3208)

[mael-hf]

• fixes issue #3208
• I created a new storage called secondaryStorage, which keeps in memory the excess images.
• the total number of images stored is 500 (as defined in src/utils/history.cpp and proposed in the issue).
• the loadMore button is always at the bottom of the UI, except when there aren't anymore images to load, then it disappears.

[mmahmoudian]

Thanks for the contribution. The code looks good and clean to me apart from one comment I made. Let's also wait and see what other devs think about this.

[borgmanJeremy]

Hello,

I started a review / testing of this. Would you be able to rebase on master? There was a minor conflict, but something in the qt6 / cpp20 transition made the calls to std::min unhappy. int and qsizetype are not compatible.

Once that is done I'll do some local testing and merge (if no issues).

Thanks and sorry for the long delay.