pidgin/pidgin

Use G_DECLARE_DERIVABLE_TYPE for PurpleConversation and additional cleanups

The setters for PurpleConversation are kind of crappy because of the
conversation cache in the purple_conversations api, which we'll address at a
later time.

Testing Done:
Compiled and ran locally, parted a chat and just signed out to verify no new issues.

Reviewed at https://reviews.imfreedom.org/r/613/
# Purple, Pidgin and Finch
libpurple is a library intended to be used by programmers seeking to write an
IM client that connects to many IM networks. It comes with support for Bonjour,
IRC, and XMPP, among others and there are many more networks supported via
[third party plugins](https://pidgin.im/plugins/).
Pidgin is a graphical IM client written in C, which uses the GTK toolkit.
Finch is a text-based IM client written in C, which uses
[libgnt](https://keep.imfreedom.org/libgnt/libgnt).
These programs are not endorsed by, nor affiliated with, any proprietary
instant messaging company in any way.
## Contributors
See AUTHORS and COPYRIGHT for the list of contributors.
## Building
Pidgin uses [Meson](https://mesonbuild.com) as its build system. There is a
long list of dependencies that is not currently documented here due to
differences in package names across operating systems. We plan to fix this
soon, if you have any suggestions on how to handle this, please let us know.
You can quickly build Pidgin with the following commands:
```
meson build
cd build
ninja
ninja install
```
There are quite a few options for the build that you can view by running
`meson configure` in your build directory. You can specify the options when
running meson like the following:
```
meson -Ddebug=true build
```
If you have already run `meson` you can use `meson configure`:
```
cd build
meson configure -Ddebug=true
```
## Running
Currently Pidgin can not be run from a build directory, which means you must
install it first. Once you've done that, you only need to run `pidgin3` or
`finch3`. If it fails to launch you may need to run `ldconfig` as root.
To get started, simply add a new account.
If you come across a bug, please report it at:
[issues.imfreedom.org/issues/PIDGIN](https://issues.imfreedom.org/issues/PIDGIN)
## Plugins
libpurple, Pidgin, and Finch ship with a number of plugins, but you can find
additional third party plugins at
[pidgin.im/plugins](https://pidgin.im/plugins).
## Developing
You can find more information about Pidgin development in the
[development section](https://pidgin.im/development/) of our website.