[ftinspect] Fix the glyph index selector layout when the window resizes.

Fixes #26. When the window resizes, the invisible tabs don't get notified.
Therefore, when the active tab changes, we need to trigger a manual
relayouting of invisible tabs.

* src/ftinspect/widgets/glyphindexselector.hpp,
  src/ftinspect/widgets/glyphindexselector.cpp:
  Extract public method `relayoutNavigation` from `resizeEvent`.

* src/ftinspect/panels/abstracttab.hpp: Add `relayout` virtual method.

* src/ftinspect/maingui.cpp: Call `AbstractTab::relayout` on the new tab
  when the active tab changes.

* src/ftinspect/panels/singular.cpp, src/ftinspect/panels/singular.hpp,
  src/ftinspect/panels/continuous.cpp, src/ftinspect/panels/continuous.hpp:
  Implement `relayout` method and call `relayoutNavigation` of the glyph
  index selector in `relayout`.