Add “scroll past end” option to the View menu

When enabled the height of the document is increased by the height of the view port, which means the last lines of the document are not anchored to the bottom of the view port but can e.g. be centered.

Closes #513.
Add AXDescription to image buttons
Add AXDescription to pop-ups to tell about their purpose
Fix initial accessibility of symbol pop-up

VoiceOver reads an AXValue for a pop-up button - that is the selected
item. But button's title does not set an AXValue, as it probably is
not a menu item. So change the code set to the title by setting a
single menu item with that title.

This fixes accessibility of the pop-up when accessing its value before
it is first popped up. It makes the code consistent with setGrammarName:
which works fine (that's how I discovered how to fix it).
Make macro recording an NSToggleButton

This fixes semantics of the button and thus also its
accessibility. The macro recording button represents state - more
specifically a boolean state, so it has to be a NSToggleButton (or
its specialization). Then the button is presented to the VoiceOver
user as an AXCheckbox - that is a checkbox with on and off value -
which is exactly what the button's function is. Examples of this in
system apps is e.g. the "Play" button in QuickTime Player, which is
also an AXCheckbox ("Pause" is then simply an unchecked "Play"
checkbox).
Present the document view as a group to VoiceOver/accessibility

This enables VoiceOver user to see the document view as a single
element and interact with it (i.e. inspect its subelements) only when
needed by using the VO-Shift-down shortcut (and stop interacting with
it using VO-Shift-up shortcut). This makes the element structure more
hierarchical where at the top level are only major user interface
components, so that it's easier for orientation. It also prevents
unrelated UI elements to be sequential as VoiceOver by default orders
elements by the visual order (top-down, left-right) rather then
the order in the AXChildren array. For example in this case,
the bottom status bar of the document window gets separated from
the bottom toolbar of the file browser, unlike the situation before
grouping when both bars were adjacent to the VoiceOver user.
Present the file browser area as a group to VoiceOver/accessibility
Use symbolic constants for binding properties
Improve VO for folder pop-up button