#16505 skipped every Global shortcut while ImGui had the keyboard, so
Ctrl+S, Ctrl+R and the others did nothing while the cursor was in the
text tool's box. Skip only the chords that edit text, and only while a
text field (a wx text entry, a spin control, a web view, or an active
ImGui InputText) has the focus. Skip every Global shortcut while an
ImGui modal is open on the focused canvas.
KeyChord::edits_text() defines the text-editing chords. The Global page
of the shortcuts dialog no longer says these shortcuts apply while
typing.