Rework custom reports dialog

Each saved report now has 3 actions
- run
- rename
- delete

Rename is a new feature that allows to rename a report without having to
delete/readd.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23081 57a11ea4-9604-0410-9ed3-97b8803252fd
Require custom report template names to be unique (among other custom report templates)

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23082 57a11ea4-9604-0410-9ed3-97b8803252fd