Improvements for the update process

fix update cycle happening on startup due to empty update source getting changed after settings are loaded (closes https://github.com/TicClick/steel/issues/84)
the update window won't lock a state mutex on every UI refresh
fix broken platform matching (part of https://github.com/TicClick/steel/issues/88)
add native Apple Silicon builds (part of https://github.com/TicClick/steel/issues/88)

TicClick / steel