Add a command bar to help merge process

[ABegue]

• Add 2 buttons "Use ours" / "Use theirs" at the top to select all "ours" or all "theirs" in one row. In my use case, almost everything should be "use ours" except few ones so it was faster to use that instead of clicking each button individually
• Add a filter bar, allowing to display only some modification (based on the action's name)
  • is filter enabled
  • is filter regex
  • is filter inclusive or exclusive
  • is case sensitive or not (if not a regex)
• Little improvement to increase stability by not trying to delete GameObjects that are already deleted
• Merge action now takes all the space horizontally instead of having a fixed size. Don't know why but MinWidth seems to override the Width instead of just setting a minimum