Use QLatin1String for systemPlatformVersion

Fixes a build error on Windows and probably on other platforms as well.

Thanks to Christian Degenkolb for reporting it.

REVIEW: 119156
BUG: 337181
SVN_SILENT made messages (after extraction)
SVN_SILENT made messages (.desktop file)
ComposeWidget: enable/disable the correct splitter

By using "splitter", the "mailText" text area was not enabled or
disabled. By using "verticalSplitter", this does happen so all widgets
are disabled when a message is being sent.

The (relevant part of the) hierarchy of the UI components looks like
this and explains why it solves the issue:

QSplitter verticalSplitter
---> QSplitter splitter
---> ComposerTextEdit mailText

REVIEW: 119398
SVN_SILENT made messages (.desktop file)
SVN_SILENT made messages (.desktop file)
Simplify bytearray -> text conversion for body parts

This is a first step for moving the plaintext -> HTML conversion out of the
Composer -- getting rid of its dependency on libImap.
Move the plaintext formatter into UiUtils

I'm not completely happy with this location, but it doesn't look like material
for a full-blown library, so let's stick it into the place where "bits to be
used by the GUI for pretty formatting, etc" already reside.

REVIEW: 119466
Better documentation for the PartOfCommand class