A category won't be shown if there are no relevant IDs (will add the ability to add new ones later).
Part of network/neochat#565

This draws heavily on what @carlschwan did in network/neochat!1579 but I found it easier to start again and grab the bits as I needed them plus some other copying from what I did in the Space tree model.
From my current limited testing this seems to work nicely try and break it.
At its best, this worked ok-ish, though it was always missing basic features.
It's also a massive memory leak and significantly complicates the codebase.
(Which is not yet cleaned up by this commit)
Currently, it is entirely broken and noone noticed or cared enough to report or fix that.
BUG: 455984
This adds previews for downloaded pdfs and code files.


Move the itinerary model representation to it's own component and instantiate from MessageComponentModel. This starts to lay some groundwork for previewing other files.
These share the same D-Bus service name (org.kde.neochat) which comes
with a fun little addition: KRunner activation! While this is not a
problem while NeoChat is running - since it's already registered - this
becomes an issue while searching for NeoChat in something like the
Kickoff. The Kickoff (and consequently, KRunner) tries to activate the
NeoChat D-Bus service which runs our unified push parts.
This introduces a "FakeRunner" which watches closely for calls to the
KRunner interface while we're in unified push mode (or directly called
from D-Bus but not running) so it quits immediately.
The aim is to be able to use separate delegate for things like codeblocks and quotes so that they can be styled differently.

Currently, this is only to pause playback of other media when something new starts playing.
In the future, it will also be used for other things, like call ringing, etc.
This is the first step in separation, so we can focus more on the
actions in this menu, and it's not tangled up in how the context menu is
shown and displayed.
For now everything should look identical. However this moves to using a model for the content of the message and is intended to lay the foundation for improved message content representation, e.g. splitting up a text message in multiple sections and using different delegates for things like code and quotes.
Make the user search automatically. This includes a timer to ensure that we aren't constantly pinging the server as the user types, the search is started 0.5s after the user stops typing. The `PublicRoomListModel` is upgraded to work in the same manner as it was architected slightly differently.
Add the ability to search in the user directory for friends.
This adds an option in roomlist when on the friends tab and opens a search dialog when clicked. The new search model searches the user directory for the given filter term.
Pull the generic aspects from Room search and join room pages into it's own component. This is done in anticipation of using the new generic search page for a user search functionality.
- `SearchPage` is now used for the generic version with the old one being renamed `RoomSearchPage`
- `JoinRoomPage` is renamed to `ExploreRoomsPage` inline with everywhere else in NeoChat
There is also some cleanup of the code for both search pages in here.
After downloading a file, the model calls the extractor and uses the
JSON to show some basic information about the content and allows to import
the data to Itinerary. This is entirely runtime-optional; no build-time dependencies
are required and nothing changes if the extractor isn't available.