Remove the x-test language from the .desktop files

In cmake, this is not just a matter of running `grep -v`, unfortunately.

The regexp contains the trailing LF because otherwise I cannot ask it to remove
that line instead of insisting on replacing it with an empty one.

CCBUG: 332578
Do not install the x-test translations

The .qm files should still be produced when doing the build (especially the
debug one), but there is no need to install them to the users' systems.

BUG: 332578
Add support for the x-test language

Since we're using Qt's facilities for determining what language to use (based on
the technology we're using, we're a Qt application, not a KDE one), we end up
with examining the QLocale::system(). However, its return value is actually
sanitized by the C library (?), i.e. trying to use either LANG or LC_ALL set to
x-test or x_test does not work at all.

Rather than inventing a custom scheme, it's easier to just follow the KDE's
conventions for env vars, and this patch makes it follow the KDE_LANG env
variable when asked for the x-test language. We still follow the systemwide
settings, and the Qt's own locale is not changed (so you might still get
"untranslated" messages in Trojita in case they're provided by Qt itself).

REVIEW: 117429
SVN_SILENT made messages (.desktop file)
Merge branch 'kill-the-xtest'

REVIEW: 117423
Add cleartext password plugin

REVIEW: 112834
qt5: Fix linking of plugins on Qt5
Port plugin to Qt5

The class name and plugin name must be the same, and they got to being with
trojita_plugin_, otherwise stuff breaks in at least one of the {Qt4, Qt5,
plugin_loader}.
Coding style
Do not use a static QSettings instance, respect what is passed via the PluginManager
Fix default password plugin

It is important to use the same identifier at both places :)
cleartext plugin: Better human-readable description
Porting ComposeWidget to use the password plugin
Use the password plugin for SMTP password retrieval
Add qtkeychain password plugin

REVIEW: 112835
Fix Qt5 builds of the QtKeychain plugin, and port to the new APIs
SVN_SILENT made messages (.desktop file)
PasswordJob: automatically call finished() upon any "finishing signal"

Better let the machine do this work instead of the programmer.
qtkeychain: better description
Gui: Window: Port to PluginManager