Delete report function: use less ambiguous variable name

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23075 57a11ea4-9604-0410-9ed3-97b8803252fd
Return success or failure when writing report templates to save file

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23076 57a11ea4-9604-0410-9ed3-97b8803252fd
Return guid of newly-created report template to calling function

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23077 57a11ea4-9604-0410-9ed3-97b8803252fd
Define 'custom-template' property on report instances (not used yet)

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23078 57a11ea4-9604-0410-9ed3-97b8803252fd
New functions to check for custom templates
- whether a report template is a custom one
- whether a report is based on a custom report

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23079 57a11ea4-9604-0410-9ed3-97b8803252fd