Add a custom icon to the window

[Lonami]

Currently the opened game window shows a default application icon, and not a custom sphereFACE one (under Linux, haven't tested on Windows taskbar).

[Riot]

Priority is major prior to release

[Riot]

load window icons with glfw for platforms that support them, resolves #304

→ <<cset 793fdc37286e>>

[Riot]

separate window icon loader class, re #304

→ <<cset d9b9dd98a782>>

[Riot]

Removing milestone: Beta (automated comment)