[WIP] This is a first attempt at modernising this plugin.

[romainl]

• more lambdas
• use of the new "method" syntax
• use &quickfixtextfunc to format the display
• ~~really make :Doline/:Dofile wrappers around :cdo/:ldo/:cfdo/:lfdo by removing legacy code (I will probably remove them anyway)~~ remove :Doline and :Dofile
• rename some functions
• "filegroup" doesn't depend on the display anymore except for line('.')
• "preview" as well
• "preview" now also jumps to the column if applicable
• new utility functions (WIP)