457 - Disabled keyframes doesn't work well with drag and drop operations. Progress

In synfig-core, KeyframeList::insert_time, include deactivated kf while searching next.

Solve the reported bug.

But another one appear : previous deactivated kf now move during d&d operation !
Fix 457 - Disabled keyframes doesn't work well with drag and drop operations.

In synfig-studio, Widget_Keyframe_List::perform_move_kf, include deactivated kf while searching previous.

Solve the linked bug (see previous commit 24c87b8edefc6e)
Fix 672 - Keyframe loose activated status after dragging
458 contextual menus for keyframe widget indicators - progress

contextual menu is displayed with usual kf actions (add/remove/duplicate/properties)

tofix : current time is used in place of time line mouse position
458 contextual menus for keyframe widget indicators - progress 2

- remove parent "keyframe" menu to only display keyframe actions
458 contextual menus for keyframe widget indicators - progress 3

- comment unnecessary code.
458 contextual menus for keyframe widget indicators - progress 4

- add keyframe toggle to context menu
458 contextual menus for keyframe widget indicators - progress 5

- undo de21869cbab619
458 contextual menus for keyframe widget indicators - progress 6

- prepare keyframe set description
- micro optimize and rationalize activation (set_sensitive) of actions by using variables
458 contextual menus for keyframe widget indicators - progress 7

- improve keyframe actions activation
Fix 458 contextual menus for keyframe widget indicators (progress 8)

- keyframe set description
Fix 458 contextual menus for keyframe widget indicators (progress 9)

- right click on kf widget list on a kf activate the kf
Fix 185 - Right click in keyframe list
7zip is only for 64 bits
Upgrade build cygwin-mingw script to gtk3
Add missing header
Win32: Don't build MLT's gtk2 module
Legacy folder seems not to be listed at www.imagemagick.org.

Replaced with releases
Fix calculation