plugin loader: Don't pass NULL to g_task_return_error

... to avoid running into error != NULL assertion in GTask code.

Just fall through if the returned list is NULL and error isn't set;
empty results are handled below.
systemd updates: Don't return unitialized memory
overview: Never schedule more than one reload

Similar fix as commit 773b911 to avoid scheduling multiple reloads at
the same time.
overview: Only mark the cache valid when all load tasks have completed
Marshal GsApp property changes callbacks to the UI thread

Otherwise we end up calling gtk+ functions in the worker threads and
occasionally transhing data, leading to crashes.
Remove g_source_set_name_by_id calls

It turns out to have been a major contributor to hard-to-debug
gnome-software crashes. Unlike the rest of GSource, g_source_set_name /
g_source_set_name_by_id are not threadsafe and calling them from a
worker thread can lead to double frees and / or trashed heap.

https://bugzilla.gnome.org/show_bug.cgi?id=736683
details: Avoid refreshing the page when it's not visible

This makes sure that we don't call switch_to(details) in response to app
state changes when the user is actually no longer on the page.
Keep a ref on back entry focus widget

This fixes a crash when going back from Details to the Installed view
after having removed an app.
installed: Try harder to avoid scheduling more than one reload at a time

... so that the items in the installed page don't show up twice after
doing an updates refresh.
GsApp: Avoid crashing on kind changes on "core" apps

Handle newly added GS_APP_KIND_CORE enum in gs_app_set_kind. It doesn't
make sense for these to change their kind, but instead of crashing just
log a warning.

https://bugzilla.gnome.org/show_bug.cgi?id=736362
Revert "Never reuse a GCancellable to prevent a crash"

This seems to have broken the shell search provider so that it got
stuck after searching for anything. Furthermore, getting stuck also led
to the gnome-software UI not working and only reporting "No Application
Data Found" after having being launched through gnome-shell search.

https://bugzilla.gnome.org/show_bug.cgi?id=736544

This reverts commit 677f8dc650840cbd28a3c4b0460dacdbd01f8217.
Release version 3.13.92
trivial: Post release version bump
Updated Danish translation