Move string-child conversion before uid-assignment

Setting UID's to branches should happen after the plain-string typed leaves are converted to objects. Otherwise the uid for those leaves is not set and Angular outputs the "ngRepeat:dupes" error. The bug can be reproduced by changing the "expand-level" attribute to "3" on the test pages (at least on bs3/ng120)

The angular error: Error: [ngRepeat:dupes] Duplicates in a repeater are not allowed. Use 'track by' expression to specify unique keys. Repeater: row in tree_rows | filter:{visible:true} track by row.branch.uid, Duplicate key: undefined