Improve directory layout to better fit with sandboxing

Currently web-apps don't work and web extension settings don't work with the sandbox.

Since we needed to move around where profiles were located anyway this fixes #182 (closed) at the same time:

• Profiles are moved to XDG_DATA_HOME
• Web applications now have a .app file to ensure they are detected regardless of directory name
• Adblock and safe browsing data is moved to XDG_CACHE_HOME (and no longer shared since that can't work w/ sandbox)

It also stores web extension settings in a text file since the sandbox doesn't have access to DConf.

TODO:

• Fix web application tests
• Move web extension settings back to XDG_CONFIG_HOME (and possibly consider if anything else belongs there)