GUI context menu improvements (incl. "reply to chan")

Enables multiselection in all tabs and disable only items unfit for current selection
- batch enabling/disabling for identities, subscriptions, and blacklist items
- batch copying for identities, subscriptions, adress book, and blacklist items
- batch deletion for subscriptions, Address Book, and Blacklist (asks for confirmation if you want to delete more than one address at once)

Unification of Inbox and Sent tab context menu:
- Recipient and Sender submenu for respective (batch) "send to", (batch) "copy", "add to address book" and "subscribe" commands (add sender in inbox, add recipient in sent tab)
- "View HTML as formated" also in Sent tab
- "Save message as" also in Sent tab

Inbox context menu:
- Additional "Reply to Chan" command
- Additional "Mark read" command in Inbox

Sent context menu:
- Additional "Send another message" command for sending with the same FROM and TO and quoting the original message

In identities tab:
- "send to this chan" command (filters out own adresses; filters out addresses already in recipients)
- "send from this address" command
- additional "join / create chan" context menu command and button

In Identites, Subscriptions, Address Book and Whitelist:
- "send to this address"
- "subscribe to this address"
- "Add to address book"

# todo
- add to whitelist/blacklist context command
- multiselection for subscribe?
- multiselection for add to addressbook?