MainFrame: don't ignore WM_MOUSEWHEEL

Fixes #4070
MainFrame: adjust the fullscreen size on move
MainFrame: adjust the fullscreen size on open

Fixes #4082
MainFrame: tweak overridden Alt key behaviour

When menubar has focus, we don't receive WM_SYSKEYDOWN messages, only
WM_SYSKEYUP messages. Overriding only show menubar behavior is
reasonable compromise.
Stop mouse hider when entering menu modal loop

We don't receive WM_MOUSELEAVE message until the loop exits
otherwise
Controls: properly remove unused hide ticks

Otherwise we may stay subscribed to periodic timer without a need to
PPageFullscreen: dirty when changing hide policy
Don't read media flags in "delay show not loaded"

They can be whatever
Seekbar: fix possible memory leak
PlayerBar: better way of detecting context menu
PlayerBar: more strict access specifiers
MainFrame: change zorder after reposition

Refs #3980, #4037
MainFrame: show menu after reposition

Refs #4004
Don't MoveVideoWindow() during fullscreen toggle

Refs #4004