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
Configure password plugins from the settings dialog

Bugs added by jkt.
cleartext plugin: compact description looks better
SVN_SILENT made messages (.desktop file)
Ubuntu: Fix for dual use mailbox's

Enables the viewing of dual use mailboxes. Clicking on the progression will show any nested mailbox's
Clicking anywhere else will show the message list for current mailbox.

BUG: 333166
REVIEW: 117446
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.