Resolve "Loading custom metainfos updates the global metainfo (through the `_allInheritingSections`)"

Closes #1063 (closed). A cache in the store is used to keep track of all known cases of inheritance. Also fixes some bugs in the Metainfo browser due to the fact that there can be several sections with the same name in the root lane under the "other root sections" group, and under the "inheriting sections" group.

Also fixes gui warnings (Warning: Each child in a list should have a unique "key" prop.), and some changes to improve the gui test stability.