Quite a bit of code in the end - mostly boring boiling one though.
Note this also introduce the 'migration' from a single 'CHANNEL' area in file browser,
to a 'TOOLS' one, with dedicated 'TOOL PROPS' one for operator options, like in 3DView.
This is in preparation for more complex needs of real asset stuff, where tabs will be
definitively needed - and also because since operator panel remains defined in C, it
would otherwise always be on top, which is not good!
Had to do some tweaks to a few side areas too, like e.g. enabling a more advanced
behavior of tooltips, to allow keeping display of the full path of the bookmarks there.