Merge branch devel into master

See merge request !59

Changes at GNUKhata version

Version changed from 4.25 to 4.50 at main and about page. And License is updated

See merge request !58

in mainshell.js file license is updated.

Merge branch devel into master

See merge request !57

Customer Supplier Module

Insert key shortcut works for save in Add Customer/Supplier.   
Also fixed accesskey for save.  
Customer/Supplier Role get selected automatically after save.  
Current Customer/Supplier gets selected after Reset.  
closes #849

See merge request !55

Created changelog for version 4.50

See merge request !52

Final version of Data Import / EXport works seamlessly with GNUKhata

See merge request !56

Modified License .

Modified license in all py files.

Modified license in few js files.

See merge request !53

Edit Invoice functionality and alerts for attachments not in JPEG format in Voucher, Invoice and Delivery Challan

closes #830   
closes #823  
closes #833  
Please see Merge request in core before merging this.

See merge request !44