Added html accesskey s to the save button.

Enables saving an entry using SHIFT+ALT+S in Firefox,
SHIFT+ESC, S in Opera etc.
Increased the size of the note editor textarea.

This is a candate for externalization into the config file,
but I chose the quick minor change for now.
Autofocus textarea when editing notes.

This requires javascript but on the other hand the application will
still work in browsers without JS support. It's just a nice gimmick
that makes life easier.
Improved category preselection and upper menu.

* When writing a new note the previously visited category will now be
  preselected. Rationale: it feels natural for some users to first select
  (read) a category and then hit the "write" link to put a new note into this
  category. The behaviour when editing an existing entry did not change: there,
  the entry's current category has always been preselected.
* The "read" and "write" links in the upper menu are now context-sensitive,
  i.e. they lose their hyperlink character when active.