Now Inset::dimension is only an access to cache

Now Inset::dimension is only an access to cache

Up to now Inset::dimension was either a helper function to access
CoordCache, or... something else. This created problems to properly
use it.

In particular, the definition of InsetText::dimension created problems
for child classes. Removing this definition (actually renaming it to
dimensionHelper) allows to streamline the code.