Improvements to Linux notification manager

The notification spec handles -1 and 0 as special values:
-1 -> representation for "undef"
 0 -> infinitely
 values above 0 represent the timeout in ms.

The notification messages should disappear again and with 0, this insn't
the case. Setting it to -1 lets the notification manager decide the
timeout and displays the notification like all others.

The current notification spec says image-data should get used instead of
image_data.

fixes #305

The latter will not always return a valid pointer.

fixes #363

Add method to convert matrix content URIs to http links

Added FindOlm.cmake script from mtxclient to find generic libolm packaged without cmake support. (#370)

Signed-off-by: Vitaly Zaitsev <vitaly@easycoding.org>

* Working D-Bus desktop notifications

* Remove return type on constructor

* Fix the Windows placeholder class

* Fix wrong variable name

* Fix windows and macOS versions of notificationsmanager

fixes #215

Requires backfilling which is not implemented.

fixes #365

fixes #351

fixes #346

The room list would be hidden only through window resizing.

Includes documentation & dependency/call graphs

Ctrl-O -> Chat
Ctrl-L -> Room list

fixes #160
fixes #163
fixes #187
fixes #127