Continue WebTree implementation. Replace 'ShowHeader' and 'ShowRowNumber' by a 'Header' property in WebTable.

[GB.WEB.GUI]
* NEW: WebTable: Replace the 'ShowHeader' and 'ShowRowNumber' properties by a 'Header' property.
* NEW: WebTable: Draw the vertical header the same way as the horizontal header, and make it sticky.
* NEW: WebTable: Replace the 'DblClick' event by the 'Activate' event.
* NEW: WebTree: Implement the 'Activate' event.
* BUG: WebTree: The "select all" checkbox now works as expected.
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