Rewrite the Topic tab in the Channel Options dialog and the backing code.

Changelog excerpt:

* The Topic tab in the Channel Options dialog has been redesigned and
rewritten from scratch, featuring a much improved UI and many bug fixes:
- In place of the previous UI with a multi-column topic history list and two
distinct text fields for browsing and editing there is now only the list
and an edit field. The list has been redesigned to show all of the data for
a topic, with visually distinct headers serving to delineate individual
entries and showing the author and timestamp, above the full text for each
topic.
- Entries in the history list now sport a context menu allowing to copy the
topic text and querying the topic author.
- A new search field above the history list allows filtering it by looking
for the search string in the text, author name and timestamp of all topics.
- When an encryption key is set for a channel an attempt is now made to
decrypt all of the topics in the history, not just the current topic.
- When editing the topic any text entered past the server's maximum
allowed topic length will now be drawn in the color scheme's negative
text color (i.e. usually red). Further, if Konversation has been built
against KDE Platform v4.7.0 or higher, a warning is shown at the bottom
of the text field explaining the limit and offering the option - via a
button - to delete the excess text.
- The text cursor is now automatically placed at the end of the edit field,
making the common use case of adding to the topic more convenient.
- The location of the splitter handle inbetween the history list and the edit
is now remembered across all Channel Options dialogs and application
restarts.
- Storing the topic history for each channel now takes up less memory, and
general efficiency in handling with topic data has been improved greatly.
- Fixed a bug causing redundant entries to accumulate in the topic history
across reconnects when the last entry in the history corresponds to the
topic given by the server at rejoin.
- Fixed a bug causing the "Undo" action in the edit field to occassionally
stop working, making it impossible to return the field to the unmodified
state in which its contents sync to the selected history list entry.
- Fixed a bug causing the topic not to return to its encrypted form when the
decryption key for a channel has been deleted.
- Fixed a bug causing the author of a topic to sometimes be shown as the full
user mask instead of consistenty showing only the nickname.
- Fixed a bug causing the "Unknown" placeholder that is used when the author
of a topic is unknown not to be translated.
- Fixed the incorrect tab key order in the Topic tab.
* The size of the Channel Options dialog is now synchronized between the dialogs
for different channels and remembered across application restarts.
* The widths of the columns in the Ban List tab in the Channel Options dialog
is now synchronized across all Channel Options dialogs.
* Fixed a bug causing the topic at the top of channel tabs not to return to its
encrypted form after the decryption key for a channel has been deleted.