Pcbnew: board editor: allows pad edition by hotkey 'E' (was accessible only by mouse button right click)
All: minor code cleaning and very minor bug fixes.

Because now there are 2 formats, board backup files extensions are .brd-bak or .kicad_pcb-bak instead of the old .000 extension, depending on the initial file format.

Pcbnew: fix bug:  missing call to SetFileFormatVersionAtLoad after loading a board, in legacy format.

fix a fairly serious bug which would leave SOLDERMASK_LAYER_FRONT layer turned off in PADs, when loading *.kicad_pcb files

eeschema: make new netlist format (S expr netlist format) the default for Pcbnew netlists in dialog netlist.
Also move netlist_control.* to dialogs/dialog_netlist.*, to be consistent with other file dialogs.

Pcbnew: add a dialog to load a legacy file in "old" projects, when the .kicad_pcb file is not existing, and the correspoinding .brd file exists.
Fix also an issue in pcb_parser when a netclass name is just a number (like a track width as name)

fix pcbnew file save as problem, if previous file by same basename did not exist.  Show default file ext.

Add KiCadPcbFileExt global.

Pcbnew: Drc dialog: fix a old very minor bug.
Added: single click on an item in marker list moves the board  graphic cursor on the corresponding marker location on the board
Double clicking is not modified.

Also a minor cosmetic enhancement

Eeschema: Add hotkey 'U' to edit reference. Better filtering for  hotheys V, F, U: now the corresponding field is selected without selection list between component and the field.

All: Fix an usually unwanted behavior in kicad: When closing a dialog (mainly ERC, DRC and recently a component selection by the library viewer on a double click) the mouse release left button event (which is seen by the parent after closing the dialog, becuse it is actually made in the parent) generate an unwanted command (item selection, component place ...)
This event can be now skipped if needed by calling  EDA_DRAW_FRAME::SkipNextLeftButtonReleaseEvent() from the dialog.
Very minor other fixes

Try to fix an unwanted left mouse release button event, when closing a dialog on a click or double click,
and therefore when the mouse butoon is released in the parent window
Minor other fixes.

window which would cause the part to be parked rather than staying
in drag mode.