General refactoring, and changed behaviour of 'double tap of tab'

Removed the path_mangling class, which was just working as a namespace.
Functions are now global in the module namespace, which seems sensible
enough.

Changed the behaviour of the double tap of tab, does not use timing any
more, and will always activate on the second press of tab.
Fixed bug with default keymap for Linux. Added new options.

Had accidentally used 'super+o' as default keymap on linux, when it
seems that it should be 'cntl+o'. Also added option to use default file
open behaviour, by using shift and the normal sequence.
Added command to palette

Enabled 'Find File' command in command palette. This is useful if the
keyboard shortcut fails to work for some strange reason. Also it's
generally the expected behaviour of most packages to add commands to the
palette.
Updating readme.
Removed some leftover code.

Removed superfluous code from previous double-tab behaviour.
Merge branch 'develop'