Prevent crash when Model gets deleted while a KeepMailboxOpenTask is still alive

The switch to a QPointer seems rather big, but let's hope that the overhead of
this is more than compensated for by a reduced potential of memory corruption.

BUG: 336090
REVIEW: 118671
Remove duplicated calls

These actions are already performed within abort() which we explicitly call, so
there's no point in repeating them once again from here.
SVN_SILENT made messages (after extraction)
SVN_SILENT made messages (after extraction)
SVN_SILENT made messages (.desktop file)
SVN_SILENT made messages (after extraction)
SVN_SILENT made messages (.desktop file)
Move Gui::Util::tintColor to UiUtils
Move text quoting into an extra file
Move plaintext -> HTML part prettifying into the Composer's PlainTextFormatter
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
Ubuntu: Implement HTMLification of plaintext mails

We're using the UiUtils::Formatting here to save an extra QML module import
which does not appear to be necessary.

REVIEW: 119082