Add possibility to selectively remove object tags in order to reduce file size and memory consumption.
This feature was proposed by @krticka in #2341.

Implementation proposal:

The proposed implementation follows Mapper's typical behaviour:

• operate on selected objects (thus limit removing to current map part)
• allow to undo the removal (since adding an undo step does not reduce file size and memory consumption, the option is not active by default)

If all object tags shall be removed the user needs to

• switch to every map part
• select all objects of that part
• and remove all object tags by selecting a non-existent key (e.g., !) in combination with 'not equal to'