qt: capture file properties: add a splitter

qt: capture file properties: add a splitter

Add a splitter to the capture file properties dialog. This allows for
modifying the relative sizes of the details vs comment boxes. People who
paste lots of text into the capture file comment may want a lager box
for it.

Change-Id: Id79d9f5fd7e589a2ba88aa5f16b52bb37d7c47ae
Reviewed-on: https://code.wireshark.org/review/26845
Petri-Dish: Martin Kaiser <wireshark@kaiser.cx>
Reviewed-by: Roland Knall <rknall@gmail.com>
Tested-by: Petri Dish Buildbot
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>