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
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
Configure password plugins from the settings dialog

Bugs added by jkt.
cleartext plugin: compact description looks better
Remove settings keys for IMAP & SMTP passwords

These are now tracked by the cleartext plugin, and as such they should not be
left in the Common::SettingsNames anymore.
Fix the slot name in these connections
Password plugins: tell the user what went wrong when a job fails
Inform the user about password plugin failure modes

Tested with the QtKeychain which uses KDE's KWallet as a backend. Especially
with GPG, this is rather painful.

Future work will be to make this UI-agnostic so that the QML UIs could benefit
from this work.