wallpapers/image: Move ImageSizeFinder to finder/imagesizefinder.cpp
Don't crowd the code in backgroundlistmodel.cpp
Test result
> ./build/bin/testimagesizefinder
********* Start testing of ImageSizeFinderTest *********
Config: Using QtTest library 5.15.2, Qt 5.15.2 (x86_64-little_endian-lp64 shared (dynamic) release build; by GCC 11.2.1 20220103 [revision d4a1d3c4b377f1d4acb34fe1b55b5088a3f293f6]), opensuse-tumbleweed 20220406
PASS : ImageSizeFinderTest::initTestCase()
PASS : ImageSizeFinderTest::testImageSizeFinder()
PASS : ImageSizeFinderTest::cleanupTestCase()
Totals: 3 passed, 0 failed, 0 skipped, 0 blacklisted, 7ms
********* Finished testing of ImageSizeFinderTest *********
Edited by Fushan Wen