Standardize file locations on Linux

[sanderfoobar]

GUI currently uses the following files and directories:

• Wallets are written to ~/Monero
• Blockchain is written to ~/.bitmonero
• GUI log is written to ~/.bitmonero/monero-wallet-gui.log
• Configuration is written to ~/.config/monero-project/monero-core.conf
• Desktop entry is written to ~/.local/share/applications/monero-gui.desktop
• Cache is written to ~/.local/share/Trash/files/ and ~/.local/share/Trash/info/

Quoting rnhmjoj

The idea of the XDG spec is to allow the separation of configuration files, user generate data and cache files. The main advantage is that it makes the locations configurable; also it's easier backup by simply selecting the directory (like $XDG_DATA_HOME/monero), without worrying about excluding temporary files.

I get the point about fragmentation but monero-gui is already using XDG_CACHE_HOME for the qml cache, and the log file certainly belong to this category. An "uninstall" procedure could simply be to delete: $XDG_{CACHE,DATA,CONFIG}_HOME/monero-project

This issue proposes to discuss that GUI related files could go into $XDG_{CACHE,DATA,CONFIG}_HOME (except for ~/.bitmonero) so that it follows the XDG spec.