Use ConvertLegacyConfig to check for -32000 pos

It was still possible in 0.3.21 to see (-32000, -32000) window
positions under certain circumstances (issue #135).

Solve this by using the ConvertLegacyConfig method to interpret
-32000 values in config files from <= 0.3.21 as default.

This should reset the window positions for anybody who experienced
issue #135 once they update to the new version.
Restore window pos even if start minimized
Merge pull request #144 from dvide/convert-v0.3.21-config

Use ConvertLegacyConfig to check for -32000 pos