Bug #629136: Move the code that updates the sensitivity of immutable page actions from gnc_main_window_switch_page to gnc_main_window_generate_title.

Patch by Guilherme Salgado.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23132 57a11ea4-9604-0410-9ed3-97b8803252fd