Allow to specify name of tree generated by external tool

[westram]

Motivation:

• needed by #595
• using auto-generated names like 'tree_RAxML_16285' makes it impossible to write macros which perform some work after the has been generated
• for that purpose we also need a way to wait (in macro execution) until a has been written back to ARB

Possibilities:

• add an input field for name
• add a button next to that input field generating a dated name (e.g. 'tree_RAxML_20140211_143319')
• somehow reserve generated names in ARB, such that starting multiple runs (esp. via macro) do not auto-generate identical names

[westram]

Milestone renamed