Merge master: the shortcut table, local models, and a Mac still in them

Four files disagreed, and all four the same way: master had turned things the
Mac branch wrote out by hand into one list to read from.

Shortcuts are the whole of it. master gave every binding a row in
hotkey.SHORTCUTS, so the Mac's DESKTOP_IDS is gone and CarbonHotkey reads the
desktop id off that row, which also gives the new cancel key a status line on a
Mac. Settings builds its four rows through master's _shortcut_row, and that one
now asks _install_buttons for Install and Remove, so macOS gets a combination
box and nothing to press, and everywhere else the button says the desktop's own
name. dikte.py starts the listener from the same table, on macOS whatever the
setting says: there is nothing installed for it to be a fallback to.

The rest is two imports and a paste list that lives in paste.Desktop now.

CONTRIBUTING.md

@@ -62,7 +62,7 @@ inside the chooser and read it there every time: a constant settled at import is
 one no test can stand somewhere else.
 
 The tests are split along the same line, and almost none of them are skipped.
-697 of the 737 run on any machine, including every line of the Wayland, X11 and
+892 of the 935 run on any machine, including every line of the Wayland, X11 and
 macOS backends: the programs are faked at `shutil.which`, the frameworks at the
 one function that loads them. A test class says which system it is standing on
 rather than avoiding the question:
@@ -78,17 +78,17 @@ to a chooser cannot quietly break the platform nobody is sitting at. What the
 systems owe in common is written once as a contract class and subclassed by each
 of them.
 
-The 40 that do carry `@linux_only` are the ones that would need the real thing:
+The 43 that do carry `@linux_only` are the ones that would need the real thing:
 the `/dev/input` listener, KDE's shortcut file, GNOME's gsettings. Mark a test
 that way only when faking it would leave nothing to test. A test that quietly
 stops running on the platform you are porting to protects nothing.
 
 ## What a pull request should carry
 
-A change to behaviour comes with a test for it. Adding a provider means a test
-that the request goes to the right URL with the right fields; adding a platform
-means a test for whatever the parsing of its device list, clipboard or shortcuts
-looks like. Adding a setting means both halves of `settings_ui.py`: the round
+A change to behaviour comes with a test for it. Adding a provider means a row in
+`config.TRANSCRIBERS` and a test that the request goes to the right URL with the
+right fields; adding a platform means a test for whatever the parsing of its
+device list, clipboard or shortcuts looks like. Adding a setting means both halves of `settings_ui.py`: the round
 trip in `tests/test_ui.py` is what catches only one of them being written.
 
 Match the surrounding code: it is plain Python with no framework, comments