Implement resize support for WindowWidget

- We now fill any additional background when growing larger
- Even if we grow too small, we still hold the full canvas for
  the Neovim window
- Changed the widget policy to minimum - i.e. the widget can
  now grow larger than the actual window but never smaller
Add helper method for windowWidget coordinates
Add helper WindowWidget::neovimLineHeight()
Remove split windows on layout change
Add window paint debug dumps

- Added -DDEBUG flag when building in debug mode
- Setting the env variable NEOVIM_QT_DEBUG_WINDOWPAINT causes windows
  to dump paint operations into jpeg files