allow editing messages

[r10s]

after #4694 is done:

• add jsonrpc for dc_send_edit_request()

• add the menu entry "Edit" (string global_menu_edit_desktop) to the context menu of outgoing messages that are not info/videochat and have a text and does not have html and where the chat allows sending in general (check chat_can_send(), there were discussions about having a core-api, but were discarded for $reasons)

• when "Edit" is tapped:

  • create a draft with title set to "Edit Message" (string edit_message) and subtitle the text to edit (use the same layout as for quotes)
  • right of title/subtitles, as usual, show a "X" that cancels editing and reverts to normal sending mode
  • hide "attachment" button, leave "send" button as is
  • it is okay to discard existing draft, if that is easier than keeping them
  • in case the whole text is removed, disable the "send" button as usual

• when "Send" is tapped from "Edit":

  • call dc_send_edit_request() with the new text
  • it is fine to call the function even if the text is not changed (will do nothing then)
  • go back to normal "enter message to send" mode

this UI is known from many other messengers

refer to deltachat/deltachat-ios#2611 and deltachat/deltachat-android#3601 for example code and screenshots