ExportSpriteSheet: Fix "Height" combobox to make it editable
When copy and paste layers between documents, expand the number of total frames of the destination sprite if it's needed
ExportSpriteSheet: fix getting the best fit size for long animations
Fix a bug in ui::Manager putting the focus in non-visible widgets

We've to ask for isVisible() member function because it checks for
the visibility of the widget and its parents.
ExportSpriteSheet: setup text of columns/width/height when the other fields are modified
Avoid crash by ASSERT()

I think this ASSERT crashes the program because they aren't called from
the main thread, but I'm not quite sure.