[push] Simplify the cell editor & remove the hack

* There is no need to extend DefaultCellEditor: AbstractCellEditor is enough
* getCellEditorValue() can return the whole
  RepositoryWithBranchPanel, and actually should do it, because it is
  the value stored in the RepositoryNode.
[push] Fix vertical size of the editor which is not high enough by default
[push] use default border for the target text editor.
[push] remove selection from the text on focus lost

E.g. in Git, when selecting remote from the popup.