kate: make size of persistent toolview also persistent

... rather than having QSplitter change this when switching non-persistent bars

So, when the user has chosen to make a toolview persistent, it is likely that its size is then also set in some convenient way. If a different non-persistent tab is then switched, one might hope that the switch occurs in the "remaining size". However, since some widget (un)hiding happens, QSplitter redistributes space and so the persistent tab ends up with some different size, and then again another size when switching non-persistent, and so forth.

This change aims to also persist the persistent tab(s) size during a switch.