[snipmatch] Bug 444353: Make Snipmatch editor extensions discoverable

This change adds an extra header to each of the editor pages.
The header contains an action that opens the p2 discovery dialog.

Change-Id: I5ad469a0041b7bd5d51a9be332a19540e4c58178
Signed-off-by: Stefan Prisca <stefan.prisca@gmail.com>
Signed-off-by: Andreas Sewe <andreas.sewe@codetrails.com>
Signed-off-by: Johannes Dorn <johannes.dorn@codetrails.com>