Make Partition Information dialog warnings selectable (#705596)

Allow all partition warning messages to be selectable and copied.  Merge
all the messages into a single Gtk::Label widget so that they can be
selected together.

Use a blank line between individual messages so that each message can be
distinguished. Therefore each message should be formatted as one or more
non-blank lines, with an optional trailing new line.  This is true of
GParted internal messages and probably all external messages and errors
from libparted and executed commands too.

Bug #705596 - Partition Information Dialog - let user copy warnings