To coordinate the MSI installer with Jami's close-to-tray feature,
a custom action was used to kill QtWebEngineProcess and Jami, prior
to the file installation step. The close-to-tray feature makes it
so the only way to terminate the app is via the systray context
menu.

This patch harnesses the IPC mechanism used by the run-guard, and
adds a command-line option('--term') used to signal the secondary
process and provoke graceful termination.

The benefits are:
- the app can save data before closing
- system tray icons aren't left dangling after updates
- QtWebEngineProcess is guaranteed to be terminated during install

Gitlab: #654
Gitlab: #543
Change-Id: I79421eeab49c9ec0826010af99a364471bb81d1a