Move ~/.lmmsrc.xml to canonical config location

[sqrvrt]

Moves $HOME/.lmmsrc.xml to $XDG_CONFIG_HOME/lmms/lmmsrc.xml (and similar dirs in other platforms) to conform to modern standards.

Currently in this PR, if file doesn't exist in new location, old one gets copied, and if directory doesn't exist it gets created on startup instead of on write like the file does. Should i move it? Creation also silently fails if directory is somehow unable to be created, though i'm not crazy enough to test if writing a file would still emit a warning.

Worthy of note, since it's now a proper config file, the leading dot is removed. Instead of copying old file could be moved. Current behavior is for testing convenience.

[Monospace-V]

I would copy and not move because in my case (and I think at least some other users do this too) I'm running multiple versions of LMMS. I think this could be more likely in the future if there are changes that break backward compatibility. I know a lot of other users who switch from nightly back to alpha, etc, or try out nightly and go back to stable.

[sqrvrt]

My main argument for moving was just somewhat of a clarification that file being observed has moved, but i see the problem. Would probably be good to get the point across in some way though without reading the patch notes, maybe a popup after file copying.