Cleanup the caret geometry code

Cleanup the caret geometry code

Intended changes:

* code is shorter and cleaner

* caret scales better with zoom when cursor_width=0: completion
  indicator, l-shaped cursor...

Details:

* Rename BufferView::getPosAndHeight to getPosAndDim because ascent is
  needed too and width could in the future be set depending on font.

* Get rid of rect_ in CaretWidget and record a Dimension (and y value) instead.
  Remove also caret_width_ and slant_, replace rtl_ with dir.

* Make CaretWidget members public and lose the trailing _.

* change CaretWidget::update to read its parameters from current bufferview.