lspclient support for server progress notification

Since a server might use this for anything it chooses, it is probably best not to show this in a too intrusive (pop-up'y) way. As such, the provided progress report/message is simply routed/signaled towards kate's outputview. The latter is slightly adjusted to allow a subsequent message to replace a previous one. Such is reasonable for a progress report message, and works out well for e.g. clangd's index progress report.