Improvements for the date and priority action buttons, closes #2386

[wshoy]

In this PR:

• Date and time
  • Made setting dates easier by not disabling the "Pick Date/Time" buttons and changing the current date checkbox behavior, fixes #2386
  • The format checkbox also disables the "Pick Date/Time" buttons
  • The current date checkbox updates the date preview
  • Removed the word "Always" from the current date checkbox description (retranslation needed) https://github.com/gsantner/markor/issues/2386#issuecomment-2271733903_
• Due date
  • Used "Due date" instead of "Current date" for the due date action button text
• Priority
  • Added long press behavior to set/unset priority from the previous line
  • Made the highlighting colors equally bright and replaced yellow with magenta