Ignore empty string

When a string is empty, we currently create an entry for it, check
the validity of string and early return if it doesn't contain any
data.
Add 'Merge all' functionality

Add a new menu item to merge all the clips in the list, in the
order they were copied, adding a new line for each item that
was merged.
Merge pull request #42 from denizt/dev

Add merge all, and fix the empty item bug