same as in 00ab193e

replace the app icons in images with the new version with extra margin
use the 256px icon instead of the 64px one in src/core/CMakeLists.txt
rm the unused 64px icon

According to qml profiler, the binding in Loading takes a lot of time, so replace it with one time assignment

It avoids multiple recomputation if it is found

for generic packaging, use 256px png icon with empty margins
exported from images/gcompris-qt.svg
(to comply with https://docs.flathub.org/docs/for-app-authors/appdata-guidelines/quality-guidelines/#reasonable-footprint )

add branding colors
(to comply with https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/quality-guidelines#has-primary-brand-colors )

make summary shorter and with sentence case
(to comply with https://docs.flathub.org/docs/for-app-authors/appdata-guidelines/quality-guidelines/#not-too-long-1 )
and https://docs.flathub.org/docs/for-app-authors/appdata-guidelines/quality-guidelines/#no-weird-formatting-1 )

to comply with flathub "Reasonable footprint" App Icon guidelines,
make a version of the app icon with empty margins.