Add engine/mode unit

Contains generic mode related routines.
Make partial use of engine/mode

Transfer isn't over yet.
Add some spaces and comments to headers of modes
Remove modes/modes:get_mode()
Add engine/mode:vle_mode_is()

Checks that currently active mode is the mode.
Use vle_mode_is() where applicable
Add functional style macros
Simplify modes/modes:modes_post()

Yes, by using GOTO.
Use functional macros with vle_mode_is()
Introduce primary/secondary modes concept
Fix views redraw in menu mode
Merge branch 'fix-view-redraw-in-menus'

Fix recently introduced issue with unexpected redraw of views in menu
mode.

Added engine/mode unit.

Introduced concept of primary/secondary modes.