lspclient support for server progress notification

Merged Mark Nauwelaerts requested to merge mnauwelaerts/kate:lsp_work_done_progress into master

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.

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