Based on the corresponding macOS code. Work in progress. The image
support ifdeffed out still (because it uses some macOS specific APIs
for which I couldn't right away find the equivalent iOS ones).

I made it much simpler than the macOS code. I dropped the keeping of a
local in-process clipboard completely. Firstly, as far as I see, the
iOS clipboard API (UIPasteboard etc) does not even offer the
possibility to separately offer some formats and actually provide the
data on request. Secondly, we must be prepared anyway that the system
can kill an iOS app at any stage while the user is using some other
app, so we need to make sure everything that is copied goes onto the
system clipboard right away anyway.

I had to disable the copying of HTML to the clipboard as that lead to
a mysterious assertion failure. See comment in
DataFlavorMapper::openOfficeToSystemFlavor(). But RTF seems to work
well, too. I assume RTF is what gets used for cross-application
copy/paste (and cross-device, even, through Apple's Universal
Clipboard thing, where you can copy/paste between your Macs and iOS
devices on the same network).

I am not sure how relevant the various application/x-openoffice-foo
formats are.

Change-Id: I174495e33d86fc3990996c229243c05d6cbfcda7