explicit convert to nativeImage to stabilise trayIcon on Windows [Electron]

[t3chguy]

fixes that trayIcon.setImage(newFavicon); sometimes threw:

(node:10936) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 3): TypeError: Error processing argument at index 0, conversion failure from C:\Users\t3chg\AppData\Local\Temp\win32_riot_icon.ico

because even though the API claims it accepts a string sometimes it just fails to want to causing a desync between the number of Unread rooms and the number of Unread rooms in the Favicon.
Also pre-build the defaultIcon for reuse

Signed-off-by: Michael Telatynski [email protected]

[dbkr]

Mmm, nice how it will only read ICOs from a file :/