Move platforms list to qt5

Currently list of platform configurations and all tested features is kept in Coin repository. That is mildly speaking suboptimal, because:

• adding a new platform or modifying existing one is not tested
• only people with access to Coin repository may change the list
• there is a risk of major regressions that blocks the whole CI
• lack of visibility of which platform is tested with which additional features
• the list is split per Qt branches and it is limited to major releases, that essentially makes it unusable for anything else then Qt5 and we can not differentiate between for example 5.8.0, 5.8.1 and 5.8

This change would also speed up releases. Currently we have a bigger set of configurations when building Qt5 then when building qtbase. That is to limit resources that we use for every integration. This approach is great for dev, and in general during periods with high load of changes, but it may slow down us when integrations happen just before the release in 5.x.y branches. Imagine such case:
Qt5.13.22 is almost releasable, there is just one minor bug in qtwebengine. The fix has to go to qtwebengine and integrate which is ~4h. Then we need to update Qt5 with the new version of the module, which triggers additional configs build which takes another 4h. Then we build packages which is ~2h. So it is ~10h in an optimistic case. So if we have qtwebengine 5.13.22 branch build against all configs right away during the module build just in the release branches, then we would reduce that time by ~4h

Note: One thing that needs to be changed when this move is complete is the way we are generating the instruction testdata in coin. Instead of tracking qt5 branches, we'll have to track specific qt5 sha1s, to allow for stable platform configurations. Essentially testdata currently covers the platform configurations as well as the build instructions. After the move it'll be limited to testing the build instructions, the platform configs are covered testing wise through full builds and test runs in qt5.git.