Reuse a ca_context for playing sound instead of creating one each play.
Reuse a ca_context for playing sound instead of creating one each play.

* src/hildon2/modest-platform.c (modest_platform_play_email_tone): instead
  of creating a new canberra context every time we play sound (and destroying
  it without making sure we have finished using it), we reuse one.

community-ssu/modest,community-ssu/modest,community-ssu/modest,community-ssu/modest