change check to `messageListView.sectionBannerItem != undefined` so that the command line isn't spammed with qrc:/RoomPage.qml:263:13: Unable to assign [undefined] to bool
This converts the emoji dialog in the chatbar to be the same popup as for reactions. This includes:
- EmojiPicker and ReactionPicker were already similar and are made identical, as such ReactionPicker no longer needed
- Emoji dialog used for both reactions and chatbar emojis
- Add some parameters to allow for different use cases (include custom emojis and whether selection closes the popup)

Set the focus to the chatbar after clicking the jump up/down buttons so that they don't stay highlighted.
Also remove messageListView.headerItem.height as part of the down button margin calc as it no longer exists
BUG: 456075

This could be helpful for some working use-case: When user want to mute a channel temporary and want to un-mute it later, the icon could help user know the mute status at a glance so they won't forget to un-mute the channel.
This icon will only be shown if the channel is muted, and have no unread message and the item is not hovered, so it won't get in the way when user have unread messages.
- Update from deprecated readMarkerEventId to lastFullyReadEventId for quotient 0.7.
- Removed duplicate code for marking read from the read marker delegate.
- Make sure that mark as read only trigger at the end of the timeline
- Allow the read marker to be highlighted when jumped to
Move the typing indicator out of the listView header so it doesn't resize the view when appearing and disappearing causing all messages to be marked as read
BUG: 457666
Ensure that in the small number of cases where the room page up down buttons overlap messages due to the screen being thin that they are always on top
BUG: 461718
Update the base item in the timeline container to be a column layout. This means that all the items can be laid out automatically without the need to set lots of manual settings and anchoring.
The overall height calculation for the delegate is vastly simplified (in fact it is removed) which deals with the fact that there were still instances where the manual calculation didn't work e.g. a delegate with a reaction followed by another message from the same user didn't give the correct bottom margin (see below)
before:

after:

This also improves upon the recently changed hover highlight behaviour. The previous patched moved it to cover the avatar as well as the bubble however it also covered the section and reaction when present which didn't look good. The highlight now only covers the avatar and bubble
before:

after:

This also cleans up some of the margins in both bubble and compact to ensure consistency.
**Updated**
Add a section label at the top which shows the date label of the next section up. This means that the user will always be able to see the date of all messages on screen.

From the feedback given I've added a background at the top. I also added an underline to the heading which applies both at the top and in the listView since they use the same component. I added it originally for the top because I felt it looked a bit weird having messages appear from behind a heading background the same colour as the listView background.
Note: I know the gaps between messages are not right. I had to set the spacing in the listView to 0 to prevent itemAt returning null. I plan to add it back in as part of the delegate code before it would be merge.
Fixes BUG:454880
Initial work to create a model so that new servers can be added to the list.
The model will also check if the server is valid before allowing it to be added.
Implements network/neochat#11
### TODO
- [x] Add functionality to cache added servers