Auxiliary symbol properties

[dl3sdo]

Mapper sometimes needs to handle symbol properties that are not part of Mapper's own set of symbol properties and that need to be applied when importing objects.
Example: horizontal and vertical text alignment is a symbol property in OCAD but an object property in Mapper.
Mapper uses auxiliary data structures to temporarily store those symbol properties for later applying them on object import.
This change adds a QVariantHash element to the Symbol base class that allows a flexible and type safe storage of auxiliary symbol properties.
These auxiliary symbol properties are neither saved to a map file nor loaded from it.

The second commit applies the auxiliary symbol properties to the DXF import where text object properties like the text itself, alignment and rotation were previously imported by putting them into a string and transporting the string via the description property of the related text symbol.
Besides being an awkward design the approach suffers from multiple deficiencies: the auxiliary string was not removed from the description field, the rotation angle was rounded (e.g., -17.2 became 20) and the way of constructing and parsing the string was unnecessary complex.
Note that this commit replaces #2135.

The third commit applies the auxiliary symbol properties to the import of OCAD maps.
Auxiliary symbol properties are replacing the text_halign_map and text_valign_map hashes that were used to store OCAD's symbol properties for horizontal and vertical text alignment for being applied later to text object import.

This PR is a POC to demonstrate a possible way of preserving symbol/object related properties until object creation.

Auxiliary symbol properties could be considered with respect to #2082 and #1639.