Revert "w32_common: don't override alt+space"

This reverts commit d859549424174179768fcd0310c75823a5ff7cb1.

Going to apply the alternative fix through PR #1256, which came just
some seconds after pushing the reverted commit. The reverted commit
was reported as not actually working.
w32_common: open window menu on Alt+Space

Since mpv doesn't call TranslateMessage, this must be done manually.

Should fix #1254