[push] remove unused method
[push] More correct names

RepositoryWithBranchPanel is not a renderer, but an editor.
At least, remove "implements" and rename the method.
[push] Don't nullize border in renderer
[push] Change loading icons to the standard ones, fix widths

Let the LoadingIcon occupy the same area as the JCheckbox does
to avoid flickering when loading icon is changed to the checkbox.

Keep an empty icon to occupy the same place for the single repo case.