panel
- Corrected handling of scrollability in layer panel (fixes 1336996)

- resetting relative coordinates
- switching units
- not official: changing transparency for layers (works only for main layers, does not work on vias/pads/etc.)

Screen size is saved in VECTOR2I instead of VECTOR2D.

Added a few settings to make the layer settings more similar to KiCad defaults (moved vias to VIA_THROUGH_VISIBLE, added a few rules for required layers).

TOOL_EVENT message is supposed to contain string as parameter.
Added missing header for class_drawsegment.h (KiROUND).
Renamed SELECTION_TOOL::containsSelected() to SELECTION_TOOL::selectionContains().

Fixed a missing group clearing and update for restored VIEW_ITEMs.Fixed a missing group clearing and update for restored VIEW_ITEMs.

- VIEW_ITEM::ViewUpdate() does not update items immediately. Now it marks them to be updated and the real update occurs on the next rendering frame.
- VIEW::InvalidateItem() made private.
- VIEW_LAYER::enabled -> visible
- Some functions moved to header files.

Added REASON enum for Reset() function, so tools will know why a reset occured.
Fixed SELECTION_TOOL (it was bailing out, when a new board was loaded and some items were still selected).
Added removal of VIEW_ITEM groups after changing layers and removing items.

Fixes the case, when waste is never greater then the worst case and seeds assignment does not occur.