improved robustness of Printing menu during a dialog with Marlin

[digant73]

IMPROVEMENTS:

• improved robustness of Printing menu during a dialog with Marlin (e.g. printing from onboard SD). Now when a dialog with Marlin is ongoing (e.g. M600) pressing on the "Resume" button will be simply ignored and an error message "busy processing, please wait..." is displayed on the notification bar just to inform the user that a dialog with Marlin is ongoing and it must be first completed.
• config.ini has been updated reporting Marlin 2.0.8.1 as the suggested minimum version fully compliant with TFT fw

PR STATUS: ready to merge

NOTE:

1. bug [BUG] Print pause not working with REPRAP_DISCOUNT_FULL_GRAPHIC_SMART_CONTROLLER disabled MarlinFirmware/Marlin#21813 needs to be fixed in order to cover all the possible Marlin settings
2. PR fixes #21813 MarlinFirmware/Marlin#21828 fixing the above bug on Marlin must be merged on Marlin

EDIT 2021-05-16:

• Marlin PR #21828 is present in the new official Marlin 2.0.8.1 so this PR is ready to be merged now
• config.ini has been updated reporting Marlin 2.0.8.1 as the suggested minimum version fully compliant with TFT fw