Refactor Schematic::sizeOfSelection() and its usages

[wawuwo]

Hi!

Nothing fancy comes with this PR, just a bit of refactoring to replace a recurring pattern (pseudocode):

for (comp : all_components) {
  if (!comp->isSelected) continue;
  ...
} 

Member function Schematic::sizeOfSelection() is renamed to currentSelection and is made to return not only selection bounds, but also containers with pointers to selected elements, which enables to iterate over selected elements directly:

for (comp : currentSelection().components) {
  ...
} 

[ra3xdh]

Merged.