adapt to new saved-messages api

[r10s]

since deltachat/deltachat-core-rust#5606 we have a new saved-messages-api; to benefit from that on android, the following steps are needed:

• make dc_save_msgs(), dc_msg_get_original_msg_id() and dc_msg_get_saved_msg_id() available in java

• when long tapping a regular message (not info ¹), show save/unsave menu option. in case of android, this should be a ★ in the action bar, directly accessible (not the overflow menu). in self-chat, the "unsave" button should be shown unconditionally, so also for message saved otherwise. an unsave-confirmation is only needed if there is no original message

• for every message shown in a chat, check if it was saved. if so, show a little ★ beside the date (screenshot)

• in self-chat, for incoming messages, show group avatar and sender name, this is a simple one-line-change as drafted already at 75e9984

• add a button to go to the original beside each saved message in the self-chat (screenshot)

Footnotes

1. at least for the first iteration, we do not want to show saved info messages and thelike in the "Saved Messages" chat; this turned out to be confusing and might also require adaptions on systems other than android (remember: saved messages are synced), cmp. https://github.com/deltachat/deltachat-ios/issues/2567 ↩

[r10s]

@adbenitez i am up to resurrect and adapt my old PR that covers the first 4 points. as i did that on iOS, i still have the cornercases in mind. but i would be happy if you can take care of the last one (in a subsequent PR then)