Add ability to specify range of URLs to visit

* mu4e/mu4e-view.el (mu4e-view-go-to-url): Add the ability to visit
  multiple URLs by specifying a numeric range.
* mu4e/mu4e.texi (The message view: Keybindings): Document ability to
  visit multiple URLs.

`mu4e-view-go-to-url' now behaves like `mu4e-view-save-attachment'. You
can specify multiple URLs to visit at once by prefixing the visit
command by <C-u>; so C-u g asks you for a range of URLs to visit (for
example, 1 3-6 8). The range "‘a’" is a shortcut for all URLs.
Better dialog to save attachments.

This commit replaces the call to `read-directory-name' by a call to
`read-file-name' when asking the user about the target location for
saving an attachment. This new dialog is better suited because:

1/ the user is currently saving a file, not a directory: he might want
   to complete over existing files (e.g., to replace an existing file
   with a new one just received).

2/ the current dialog was asking for a directory but then, if the user
   enters a directory, mu4e proposes him to "override" the directory
   with a file. This is not going to work obviously. In the new dialog,
   it is also possible to enter a directory name instead of a file name.
   In this case, a new file is created under the chosen directory name.
Merge pull request #312 from jschwab/jws/open-multiple-links

* add ability to specify range of URLs to visit
* mu4e: fix byte-compiler warning
* toys/mug: fix compilation with gtk+ 3.10+
Update mu4e.texi for Fedora command

The currently pescribed installation for webkitgtk3-devel on Fedora is:

    sudo apt-get install webkitgtk3-devel

Instead, it should be:

    sudo yum install webkitgtk3-devel
Merge pull request #328 from robstewart57/patch-1

* update mu4e.texi for Fedora command
Merge pull request #326 from DamienCassou/dialog-to-save-attachments

* better dialog to save attachments.
* cosmetics