replace previous summary with tidy overview widget

it occurred to us that the old disk view was fishy on a number of levels

• it had a non exhaustive filter list of bad mounts
• this then raised the question why the mounts are missing (when e.g. the user expected them to be there)
• the free space on a partition is not reflective of the space filelight will be able to determine the use of (e.g. when +x is missing on a dir that dir will be missing from the fileview)
• when talking about bind mounts (such as inside a sandbox) the two sizes will almost never add up
• when inside a sandbox potentially only one valid path is available ($home) so having the previous view was slightly weird

the grand new overview widget simply displays three buttons to get right into scanning root, home, or a custom path. root is hidden inside a sandbox.

this preserves most of the spirit of what the previous view wanted to do while scaling better and doing away with judgment calls on the edge cases such as bind mounts

BUG: 452275 BUG: 440152 BUG: 440151 BUG: 426594