Make cola-classic dockable

[shish]

cola-classic has all the features that I want in the repository-status panel (namely, it shows unmodified files, and has all files in the same tree); having to keep switching between windows is fiddly though, and seems odd given that every other bit of the GUI is dockable

[davvid]

Perhaps we can guard this behind an option. The reason is that that cola-classic launches a thread for each top-level path and thus can cause quite a bit of cpu to be eaten up during startup. If we guard it behind an option then we'll have the best of both worlds.

[davvid]

views.mainwindow: Make the 'Cola Classic' tool dockable

This adds the ability to have a dockable 'Cola Classic' tool.
This is enabled by setting the 'cola.classicdockable' configuration. e.g.:

git config cola.classicdockable true

The default is 'false' simply because it slows the down startup time
and is especially noticable on large repositories.

Closed by 0536466

Signed-off-by: David Aguilar <davvid@gmail.com>