Introduce shell-embedded message area, to avoid dialog windows

When code wants to inform the user about an issue, lots of KDevelop
code currently simply pops up a message dialog window, with all the
consequences like stealing the focus.
Most of the time the message though is just assisting information
why something has not worked and does not need to block the user instantly
from the normal interaction, instead could be digested by the user later.

This patch introduces a message area to the shell, between toolbars and
the view area. Which will auto-expand if messages are to be shown.
This is similar to the inline-message area known from KTextEditor documents,
but here applied to the scope of the shell.

Luckily the code from KTextEditor for the message area can also be reused
here (though only as fork for now), so it is well tested code.

Not all places using KMessageBox/QMessageBox for notification-like
messages can be replaced yet, as sometimes they are used when assistant
dialogs are open. That might need a second step to also ensure all such
dialogs have such areas and could then get the messages channeled to them.

This message system would also allow to be extended later to send off
messages to the workspace notification system if the shell window does
not have focus, for messages flagged as being important enough for this.
1 job for !87 with addinshellmessaging in 60 minutes (queued for 3 seconds)
Status Job ID Name Coverage
failed #28157
linux kf5-qt5 qt5.12


Name Stage Failure
linux kf5-qt5 qt5.12 Build The script exceeded the maximum execution time set for the job
The X11 connection broke (error 1). Did the X11 server die?
kdeinit5: Fatal IO error: client killed
kdeinit5: sending SIGHUP to children.
klauncher: Exiting on signal 1
$ python3 -u $CI_TOOLING/helpers/ --product $PRODUCT --project $PROJECT --branchGroup $BRANCH_GROUP --platform $PLATFORM
kdeinit5: sending SIGTERM to children.
kdeinit5: Exit.
Pulling docker image gitlab/gitlab-runner-helper:x86_64-ac8e767a ...
ERROR: Job failed: execution took longer than 1h0m0s seconds