Refactor Buffer into separate Buffer and Textbuffer objects

Some protocols require merged textbuffers that are shared by multiple
buffers and cannot be separated (similar to WeeChat's "merged buffers"
feature, but in WeeChat, the textbuffers are still separable).

This commit introduces support by splitting the textbuffer off into
its own class. This adds a small layer of indirection for some
operations.
visually show non-selected buffers that share a textbuffer
add support for removing lines in the textbuffer
Add support for buffer items navigation

For protocols where one textbuffer is shared by multiple buffers, add
a mini buffer navigation above the input line.
Add support for buffer activate messages

Supporting protocols can use this message to tell the client, that a
buffer was activated because of the command that the user issued.

The buffer is then also activated in glowing-bear.
Buffer Item Active server message

Support buffer item active message to change the currently active item buffer for the protocols where multiple buffers use a single textbuffer.

This signal gets sent when anyone (in the real client or a relay
client with "Mark as Read" enabled) changes the buffer, as it
corresponds with informational messages in the textbuffer, such as
"Now talking to: xxx"
skip non-existant buffers when handling nicklist message, they may already have been closed
Prefer buffer sharing the textbuffer when buffers get closed

For protocols with shared textbuffers, select another buffer inside
the same textbuffer (if one exists) rather than the first buffer on
buffer close.
Add more nicklist update operations

Implement the changegroup and rename commands for the nicklist.