Handle window/showMessage better

[rchl]

window/showMessage is currently handled by showing the text using window.status_message(). This is bad because:

• it is really hard to notice (easy to miss)
• it doesn't support line breaks and is limited in how long the text can be before being cut off
• in some cases the text will not even be shown at all. Probably because something else calls the API and overrides it. For example in fix: notify when typescript with no tsserver API found typescript-language-server/typescript-language-server#1120 I'm calling window.showMessage during initialize and the text doesn't show up at all.

This kind of message should be per-window and should be prominent. Given ST limitations, I think the best option would be to open a dedicated panel with the message shown there.

(Similarly the window/showMessageRequest request should also IMO use a panel instead of being shown as a popup in the active view since it's too easy to dismiss it accidentally and also it's meant to be window- and not view-bound. But that could be treated as a separate case.)