gtkimcontextwayland: Save context even when text_input isn't around yet

gtkimcontextwayland: Save context even when text_input isn't around yet

Remember the current context on focus-in even though the text-input
isn't set up yet. This helps in the case where the text-input is not yet
created but a widget already got focused. Without that the enable()
invocation in text_input_enter() woulnd't be invoked leaving the input
method disabled.

This fixes

    gtk4-demo --run=search_entry

which would initially not show the on-screen keyboard when e.g using
phoc/sway as compositor.