GtkEntry on a LayerSurface doesn't activate input-method

While playing around with AppGrid search and showing an osk on layer overlay, I noticed focusing the search entry does not activate the input method. This is also true for SystemPrompt PolkitAuthPrompt.

Without this, to activate the osk, focus changes would need to percolate 4-5 layers down and require quite a bit of plumbing.

This is related to squeekboard#57 (closed)