Add three-pane view

This patch introduces three-pane KJots interface, that consists of:

• Collection view (tree-structured view of books)
• Item view (list of notes in selected book)
• Browser/Editor

It adds a way to configure the arrangement (either two columns, or three columns), and uses UiStateSaver to save/restore state of splitters (new dependency on Libkdepim).

One of the issues was that both views have a selection, so it's impossible to figure out context for action (e.g. if I trigger "Lock" action, should it lock a note or a book, or both?). Those actions were split into pairs (LockUnlockNote, ChangeNoteColor, and same for NoteBook)

Rename action also have to guess the context (by checking which widget is focused).

It updates keyboard navigation (ctrl+shift+pageup/down can be used to go over books, and ctrl+pageup/down to go over notes)

It also removes custom "sort_by_*" actions, as sorting now can be done by clicking on headers in item view. It also adds additional fields to sort by (i.e. not only creation date, but also modification date, or size).

Screenshots

Here are some screenshots on how it looks like:

Before

After

Two columns

Three columns