bugfix: do not panic when no clipboard data

avoid double ` ` and `_ ` in note's filename

* extend sanitize-function to filter
  double space and underscore
* outsource the `sanitize`-filename function in its own crate:
  `sanitize_filename_reader_friendly`

add error popup windows for Linux and iOS

default templates: use international date format

fast note taking with templates and filename sync

Tp-Note is a note-taking-tool and a template system, that consistently
synchronizes the note’s meta-data with its filename. Tp-Note collects
various information about its environment and the clipboard and stores
them in variables. New notes are created by filling these variables in
predefined and customizable Tera-templates. In case “<path>” points to
an existing “Tp-Note”-file, the note’s meta-data is analysed and, if
necessary, its filename is modified. For all other file types, Tp-Note
creates a new note that annotates the file “<path>” points to. If
“<path>” is a directory (or, when omitted the current working
directory), a new note is created in that directory. After creation,
Tp-Note launches an external text editor of your choice. Although the
note’s structure follows “pandoc”-conventions, it is not tied to any
specific markup language.

After the user finished editing, Tp-Note analyses eventual changes in
the notes meta-data and renames, if necessary, the file, so that its
meta-data and filename are in sync again.

First stable release