fix: drop LowID-0 sources in CanAddSource - #790

… defaults (amule-project#790)

* refactor(gui): build the tray speed presets in one place

The ladder of limit presets was written out three times -- once in the
appindicator backend's submenu builder and once each for upload and
download in the wx backend -- and the two backends had drifted. The
appindicator one passes its labels straight to GTK, so "Unlimited" and
"kB/s" were never translated there, while the wx one ran both through
_(). One of them also formatted an unsigned with %d.

One helper produces the speeds and one produces the labels, so both
backends show the same strings in the user's language.

The clamp moves into the helper with them. The appindicator backend was
clamping in its two callers and then handing the result to a builder
that has to cope with any value anyway; there is now a single place that
decides what an unusable capacity turns into.

No change to the numbers offered. Raising the stale defaults they are
derived from is a separate question.

* fix(gui): spread the tray limit presets, and raise the stale capacity

The tray offered fifths of the configured line capacity, whose defaults
were 300 and 100 kB/s. On any current connection the top entry was
already a heavy throttle, which is what amule-project#788 reports.

Two changes, and the first helps everyone without touching a config.

The ladder is no longer even. Fifths span a single factor of five, so on
a fast line every entry lands high and there is no way to throttle hard
from the tray, while on a slow one they bunch near the top. Dividing by
1, 2, 4, 10 and 50 instead covers full speed down to a heavy throttle
from the same capacity -- at the old default of 300 that is
300/150/75/30/6 rather than 300/240/180/120/60, better reach downward
with the same ceiling.

The capacity defaults move to 100/20 Mbit, converted at 1024. They are
not limits: capacity is what the line can do, and the traffic graph
scales from it too. Only new configurations see this -- aMule writes
every key on save, so an existing amule.conf already carries the old
value and wxConfig applies a default only when the key is absent. That
is also why the ladder change matters on its own.

Deriving the presets from the configured limit instead was considered
and rejected: the menu could then only ever lower it, since every entry
would be at or below the cap in force, and the shipped default of
unlimited leaves nothing to scale from at all.

* chore(i18n): regenerate catalogs after the tray menu refactor