Add clear button for date and date-time form editor drawer

[gabriel-bolbotina]

Context: Currently there is no way to set a date/dateTime field to NULL once a value has been picked. Users need a Clear button inside the calendar drawer to reset the field.

Solution: Add a Clear button for date and date time form editors to set the value to NULL and collapse the drawer.

Implementation hints:

• app/qml/form/editors/MMFormCalendarEditor.qml and app/qml/form/components/MMCalendarDrawer.qml is where the changes will be made.
• Add a new signal to be fired when the clear button is tapped
• There is a signal that already supports null via its second param in MMFormCalendarEditor: root.editorValueChanged( root._fieldValue, true ) // isNull = true
• Also, you can take a look how the clear button works in MMFilterDateInput.qml lines 62-94 for filter date fields or the Clear button in Value Map/ Relation editor.

Acceptance criteria:

• The button appears on the top left corner of the calendar form editor
• Once pressed, the value is set to NULL and the form editor drawer collapses
• The design of the button follows the Figma designs that can be found here: https://www.figma.com/design/d78a4ehBBgK6MyxCTuCAqg/Mobile-App?node-id=6993-24415&m=dev