Modifications to Mousepad

[Michaël Meyer]

Hello,

I've changed a few things in Mousepad which I'd like to be merged. I believe it
is best to discuss this here first and get feedback. Here's what I propose:

* A "Justify" feature that wraps text at a given column by inserting or removing
  newline and space characters. It is roughly similar to the "joinlines" plugin
  in Gedit and to text filling in emacs (though less sophisticated).

  If some text is selected, it makes it fit between the column at which the
  selection starts and the right margin defined in the GtkSourceView. Otherwise,
  it wraps the paragraph on which the cursor is positioned, between the column
  of the first non-blank character in the first line and the right margin, thus
  preserving indention. (By paragraph here I mean a sequence of non-blank
  lines).

* In the "Open file" dialog, I added two filters "Text files" and "All files".
  Not having a "Text files" filter is annoying when navigating directories
  that contain plenty of binary files.

* I changed the way the names of opened files are displayed. If a file resides
  in the user's home directory, the $HOME part of the file name is replaced
  with a tilde. The goals are (1) to make the file path fit in less space so
  that it is more likely to be displayed in full in the taskbar; (2) to make
  the path easier to read. This behaviour might however be confusing when
  editing files as root, so it might be better to disable it in this case.

I'm open to suggestions, critics, and so on.

Cheers,

Michaël