Add computed columns, move properties

[jjeising]

A while ago we already planed to move some commonly (in the tracker) used properties to columns in tbl_ticket. A first guidance would probably be

tracker/src/Application/Model/Ticket.php

Lines 354 to 360 in b9fd435

	public static function with_default_properties(Model_Resource $resource) {
	self::with_properties($resource, [
	'Fahrplan.DateTime' => 'fahrplan_datetime',
	'Fahrplan.Day' => 'fahrplan_day',
	'Fahrplan.Room' => 'fahrplan_room'
	]);
	}

While moving code like this it may also be a good idea to add computed columns for ticket properties depending on the encoding profile. The encoding profile of a ticket and the properties of a ticket (e.g. if depends on other tickets) rarely change but lot's of joins are required to fetch related properties.

We should also define a convention for computed columns (like _is_master).