Support file paths in the execute task

Support entering a file path in the 'file' key of the execute task.
And expand variables in the path –>  Fixes #70.
Support file paths in the execute task

Support entering a file path in the 'file' key of the execute task.
And expand variables in the path –>  Fixes #70.
Add support for pcxsr installation
Merge branch 'next' of github.com:lutris/lutris into next
Fixes #85 : Broken support for relative path on main file / exe
Rename Icon view => Grid view

Rename in the code too.
GameIconView and GameTreeView classes become
GameGridView and GameListView respectively.
Invert Grid/List switch buttons + better tooltips

The switch view buttons order now match the menu items order.
Fix multiple_file_chooser
Provide Mupen64 runner
Provide Hatari runner
Provide atari800 runner
Provide Stella runner
Merge branch 'next'