Web IDE replace "openFiles" with "openTabs"
Description
Related to #222507 (closed)
Currently the "openFiles" property in the Vuex state contains an array of file objects that are the same ones in "entries", "trees", etc.
This can create some issues with keeping state entries in sync and introduced a bit of complexity... Let's remove this state property in favor of an "openTabs" which contains references to files.
Edited by Paul Slaughter