{visible/enabled}_when for Label on wx does not work on application start

see comment:
https://github.com/enthought/traitsui/pull/1544#discussion_r617672507

The problem encountered was that if we set the labels wx window to invisible at the start, then toggling its visibility never did anything (it seems like the UI never gives space for it to exist).
You can observe this by adding `self._label_when()` as mentioned in the above comment.

Currently, even if the `visible_when` statement evaluates to false initially, we show the label...
Then as soon as anything changes in the UIs context everything gets synced up and the label is only visible when the `visible_when` expression is true

Code to play with to observe behavior:
```
from traits.api import HasTraits, Int, Str
from traitsui.api import View, Item, Label, HGroup

class A(HasTraits):
x = Int()

view = View(
Label("Visible", visible_when="x != 0"),
Label("Enabled", enabled_when="x != 0"),
Item('x'),
resizable=True
)

if __name__ == '__main__':
A().configure_traits()
```
mess with the default value for `x` to either be 0 or non 0.