Move SNI's libdbusmenu-qt fork to the root directory and use that for appmenu too

Summary:
libdbusmenu-qt had a quirkin it that meant it would spawn a nested
event loop when updating a menu.

In order to fix StatusNotifierItem I ended up making a fork of the 3
small classes that we needed and applying a fix, whilst we waited for
upstream.

We also were including a complicated library+plugin that handled import
and exporting, just to do importing which is rather pointless.

When appmenu support was restored, we ended up re-linking the original
dbusmenu-qt library. The original still had the same bug, manifesting in
a whole new way of Kate's bookmarks menu automatically closing when you
click on it.

This patch shares the same libdbusmenu-qt lib we have for the SNI
dataengine with the one for appmenu.

Test Plan:
Everything compiles after removing the find package for old libdbusmenu-
qt
Menu in the Plasma applet works a lot better, and doesn't have the bug
of menus randomly closing.

Reviewers: #plasma, br...