Merge master: the modules moved into a package

Every file this branch touches moved into dikte/, so the merge is mostly the
rename following the edits. What needed a hand:

hotkey.py: master replaced the _macos()/_gnome() pair with one backend()
chooser, and this branch had added _windows() to the pair. Windows is a fifth
value of the chooser now, and everything that used to ask "macOS or Windows?"
asks backend() instead. The key is held by the running process there, so
installs_shortcuts() and shortcut_needs_restart() are both false for it, and
desktop_name() says Windows.

install.ps1 and the Windows README name dikte/__main__.py, the entry point the
Linux and macOS installers were pointed at in the same commit. The Start Menu
entry, the autostart entry and the dikte.cmd shim all come off one $entry
variable.

settings_ui.py: the shortcut tab now has a Windows sentence of its own, with
the Turkish for it. Falling through to the branch master wrote for a desktop
with no registry would have told a Windows user to check /dev/input. Nothing
covers that branch: there is no Windows Settings test class, the way there is
one for macOS.

CONTRIBUTING: the chooser it names is backend() now, and the test count is the
merged one, 1067 of 1110 running anywhere.

CONTRIBUTING.md

@@ -58,10 +58,10 @@ entry in a table, and one chooser picks between them, so a fifth adds an entry
 and a line rather than a branch inside every function. The tables are
 `paste.Desktop` (clipboard and key press), `audio.Sound` (capture, the device
 lists, and whether the far side of a meeting can be recorded at all),
-`paths.directories()` (where the settings and the data live) and the
-`_macos()`/`_windows()`/`_gnome()` set in `hotkey.py`. Keep `sys.platform` inside
-the chooser and read it there every time: a constant settled at import is one no
-test can stand somewhere else.
+`paths.directories()` (where the settings and the data live) and `hotkey.backend()`,
+which names the one mechanism a session has for holding a key. Keep `sys.platform`
+inside the chooser and read it there every time: a constant settled at import is
+one no test can stand somewhere else.
 
 Where a platform cannot do something, say so in its table entry rather than in
 the code that asks. `audio.Sound.meetings` is the shape of it: Windows offers no
@@ -70,7 +70,7 @@ there can tell that apart from an empty device list, which only means the tool
 that lists them is not installed.
 
 The tests are split along the same line, and almost none of them are skipped.
-1009 of the 1052 run on any machine, including every line of the Wayland, X11,
+1067 of the 1110 run on any machine, including every line of the Wayland, X11,
 macOS and Windows backends: the programs are faked at `shutil.which`, the
 frameworks and system libraries at the one function that loads them
 (`paste._win_api`, `hotkey._win_input`). A test class says which system it is