gui_toolkit 0.35.0

Added

• Drag and drop between applications (platform/dragdrop.odin, dragdrop_win32.odin, dragdrop_stub.odin, docs/dragdrop.md): the API is independent of the system; Windows implements it with OLE, other systems answer "not supported". A window becomes a drop target with drop_target_enable; a drag over it queues Drop_Enter / Drop_Over / Drop_Leave / Drop (position, modifiers, Event.drop_allowed) with the data in drop_data (Drag_Data: files, text, HTML fragment, URL and its title, PNG image, the app's own MIME types); the app answers with drop_set_effect (the system is given the last answer). drag_start runs a drag of the system with a Drag_Data and an optional Drag_Image, and returns the effect the target chose. drop_effect_for (the Ctrl / Shift / Alt convention), drag_data_types, drag_threshold, drop_inject (tests, remote control), drop_self_test (the round trip through the window's own drop target, no mouse). Pure helpers: drag_custom_encode / decode, uri_list_first, html_format_wrap / html_format_fragment (CF_HTML).
• Fingers and pens (platform/pointer.odin, pointer_win32.odin): Pointer_Down / Pointer_Move / Pointer_Up / Pointer_Cancel events with Event.pointer (Pointer_Info: identifier, touch or pen, primary, pressure, tilt, twist, barrel, eraser, contact size) from WM_POINTER. The system's mouse emulation still comes, marked Event.emulated. pointer_buttons, pointer_pressure, Pointer_State.
• Mouse_* events carry Event.buttons (Mouse_Buttons: the buttons held after the event), so a move during a drag says which button is held.
• tests/dragdrop: the pure helpers, the pointer details, the round trip of every format through the system (hidden window), injected drops.