This adopts the FFI API to use the new Context::new() and
Contest::drop() Rust-style APIs while preserving the semantics of the
existing FFI API. It introduces a wrapper around the context which
keeps an optional reference to the actual context, changing dc_open()
and dc_close() to create/destroy the actual reference.
This changes the events emitted on closing slightly: they only get
emitted when dc_close() was called while the context was open. While
dc_close() remains idempotent in that it can still be called safely
multiple times, the close events will only occur if the context is
actually closed.
A new context is now created by calling Context::new and therefore you
always have a valid context. This is much more in Rust style and will
allow a lot of furture simplifications on the context itself.
The FFI layer has not yet been adjusted in this commit and thus will
fail.
Function `query_row` executes query and calls callback function to process
row returned. Function query_row_col() is special case, that provides
callback function, which returns value of one particular column of row,
ignoring others.
In all cases, that particular column was 0 (first and only column of
query result), since there is no point to select more than one column
with this function -- they are discarded anyway.
This commit removes that redundancy, removing column number argument of
query_row_col() function and adjusting call sites accordingly.
Adjust call sites as apporiate:
* src/dc_imex.rs(dc_continue_key_transfer): use if-let pattern
with dc_read_file_safe() and dc_msg_get_file()
* src/message.rs(dc_msg_get_setupcodebegin): ditto
* src/message.rs(dc_get_msg_info): simplify code to print information
about file inside a message.
* src/message.rs(dc_msg_get_file): simplify function using
dc_get_abs_path_safe()
* deltachat-ffi/src/lib.rs(dc_msg_get_file): convert PathBuf to `char *`
to preserve C API
