Improve the loading time when a user opens a Flamingo Theme in edit mode

The opening of a Flamingo Theme for edition is slow by design, since there is a LESS compiler execution on the client-side.

However:

• the LESS compiler is executed several times when the page is loaded, 3 times instead of 2 (a second compilation is needed to take into account the variables holded by the theme).
• the "curtain" hiding the preview window is blinking between these computations.

These 2 aspects could be improved.

I first wanted to describe this issue as a bug called "Editing a Flamingo Theme is too slow" but I realized that, even with this improvement, it will still be slow (but a bit less ).