extend ACI to allow access to name of selected items (and other global states of ARB)

[westram]

Currently access to this information is only possible by manually hacking macro-files as described in help​.

Main motivation: When performing a macro with each marked species it would be useful to be able to access the name of the current used species, e.g. to append it to a -field as reference (see ​Mark by reference help).

ACI should be extended (via plugin as provided by #756) if called from a running ARB.

ACI-command	Information to provide
current_alignment	name of current alignment
selected_species, …	name of selected species, gene, experiment
current_tree	name of selected (of main -view)
cursor_position	current cursor position (used in editor and e.g. in insert column

Note:

• the plugin has to be added globally (changing all callers is not an option)