Move fis widgets SQL into scalar functions

Add fis_wdg_* table functions for dashboard widgets and wire them into
FuchsDatabase.sqlproj; update the invoice list procs accordingly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Fuchs_Database/dbo/Functions/fis_wdg_invoices_unpaid.sql

@@ -0,0 +1,12 @@
+-- =============================================
+-- Description:	Dashboard widget 'wdg_invoices_unpaid' (Unbezahlte Rechnungen).
+--              Called from [dbo].[fis_widgets].[sql] as: SELECT * FROM [dbo].[fis_wdg_invoices_unpaid]()
+-- =============================================
+CREATE FUNCTION [dbo].[fis_wdg_invoices_unpaid]()
+RETURNS TABLE
+AS
+RETURN
+(
+    SELECT [label] = N'unbez. Rechnungen', [value] = COUNT(*)
+      FROM [dbo].[fds__fn_unpaidInvoices]()
+);