Add KDE's StatusNotifierItem support

With Plasma Next KDE doesn't support the good old xembed systray, and so
GtkStatusIcon are "useless."

Adding a new configure option --enable-status-notifier to enable support
of KDE's interface via our small library, statusnotifier.

When used, kalu will try to register a StatusNotifierItem first. If it
fails, it will fallback to using the systray.
ALPM: ALPM_QUESTION_LOCAL_NEWER was removed

Also following constants were "re-indexed" so a recompile was due to reflect
those API changes.
ALPM: Event OPTDEP_REQUIRED renamed to OPTDEP_REMOVAL
Free memory from ALPM when needed

New API now allows/requires us to free memory of data we were given.
Update to ALPM's new event system
Support ALPM's new event RETRIEVE_{DONE,FAILED}
Support ALPM's new events for pac* files
Support ALPM's new events PKGDOWNLOAD_*

This allows to know when the download of a package file begins, and when
it ends (whether successully or not).

So we can move some code out of every progress update, to the start of
a download, or the end of it.

This will also fix a bug where if a download failed, its progress bar
would never go away (since it was based on reaching 100%). Now not only
does it go away, but the size will show up in red afterwards, to
indicate there was a failure.
Update to ALPM's new question system
updater: Add $PACNEW to cmdlines
Default siglevel from pacman.conf wasn't set

It was loaded from the file, but not se in ALPM (during checks/simulation)
kalu-dbus: Minor tweaks
kalu-dbus: Add downloadonly argument to Init()

When TRUE, the transaction will be set to only download packages. One
still needs to call GetPackages() & SysUpgrade() as usual.
simulation: Add button to download packages only
polkit: Add org.jjk.kalu.sysupgrade.downloadonly