Distinguish dropping 3D file and dropping a HDRI file using modifiers

[mwestphal]

Currently, when dropping a file on F3D window, F3D will load it has a 3D model unless its a .hdr, then it will load it as a HDRI.

This is very practical for most users but break the logical paradigm of F3D where one interaction correspond to one action.

While we will keep this interaction, having a dedicated interaction (Ctrl+Drop and Shift+Drop) for dedicated actions would be beneficial and let user drop any kind of file as an HDRI if needed.

[mwestphal]

@kidharb just comment on this issue, Ill then assign it to you

[kidharb]

I will work on this issue

[mwestphal]

Need #443