Main: fixate initial (possibly implicit) target size by extracting and explicit assigning

Not doing this was causing the target (Spark's file tree view) to auto-resize when contained children changed geometry (when the Run and menu buttons were hidden on text input in the file filter).
Enforce propagation of a value via data-bound attributes

Had explicitly push the change through, with different actions needed in Dartium and Chromium.
Merge pull request #2007 from ussuri/fix-spark-splitter

Fixate target size in spark-splitter on startup