conversationsview: support drag-and-drop to send file

Allow dragging and dropping files onto the conversations view on the
left to send them to a contact.  Handles multiple selected files too.
Remove old code for merging two ongoing calls with drag-and-drop.

files: add `foreach_file()' containing the logic for converting a
string containing the URIs into filenames, and calling the provided
callback on each filename.

chatview: use the new `foreach_file()' from files, much like
conversationsview does.  Reduces code duplication, and enables
sending multiple selected files at once as well.

GitLab: #1200
Change-Id: I87e11abe121db2d1ac7071236f788d77885fb18f