Bug 329353

On my netbook (or could be the same with large/big collections), on multiple occasions, the processing could be very slow and the interfaces gives little feedback to check if not crashed and make it more manageable, especially by treating small files first and giving back interface/interactivity quickly. I have time where initial scan takes hours and I can do nothing else...
Most of this stuff should either be default or advanced opion only/digikamrc probably

- when scanning for thumbnails (initial scan, maintenance), option to skip or delay at the end file bigger than X mb (default: 10MB?; if bigger, at first start, check cpuinfo/meminfo and if small, offer to adjust or settings checkbox)
- in preview/thumbs bar, if loading image take more than 3 sec, infobubble/message "Loading <filename>"
- option to limit kio subprocess active at the same time: had time when half-dozen kio_digikam* where fighting for my poor cpu... maybe a limit of one or two per core, especially for Atom processor
- set process and subprocess to idle priority (cpu/nice and disk/ionice) when minimized

Test up to 3.5 (lubuntu 13.10/kde 4.11.3 and before, macos 10.9).

Reproducible: Always