Snapmockit 1.3.0

Snapmockit 1.3.0 puts itself in the desktop's main menu, on Linux, from inside
the application.

New: Help > Add to Menu. One row in the Help menu writes the desktop entry,
the application's icon at eight sizes plus the scalable file, and the file type
for .smk projects, all under your own ~/.local/share and nowhere else.
Snapmockit then starts from the desktop's main menu like any other application,
and a project file opens on a double-click. The same row reads Remove from
Menu afterwards and takes away exactly what it wrote. Nothing asks for a
password and no system directory is touched.

It works from the AppImage and from an installation from the Python Package
Index. The Flatpak installs its own entry with the package, so its row says so
and writes nothing. The first start of a form without an entry offers it once,
as a message that does not block; the Help menu row is always there.

The AppImage is asked one question first: whether to copy itself to
~/Applications/Snapmockit.AppImage and point the menu entry at the copy, which
is the default, or to point the entry at the file where it sits. The copy costs
122 MB and means the entry survives a tidy-up of your Downloads folder; a later
release replaces that one file and the entry stays. Removing the entry offers to
delete the copy as well, and never deletes the file you are running.

If the entry appears but its icon does not, the running desktop has not
rescanned its icon folders yet: restarting the desktop shell (Ctrl+Alt+Esc on
Cinnamon under X11) or logging in again shows it.

With a Flatpak installed as well, some desktops — Cinnamon among them — list
two Snapmockit entries, one of them marked as the Flatpak's, although the
desktop entry specification makes two files of one application id a single
entry. Add to Menu says so when it finds another form's entry.

The README's nine-line shell recipe for doing this by hand is kept, under "By
hand", for anyone who would rather.

Everything else is unchanged. The AppImage, the Flatpak bundle, and the wheel
and source distribution on the Python Package Index are attached or published
as before.